This study compared young adults’ levels of anxiety toward COVID-19, their depression tendency, social support received from others, and SWB levels from May 2021 to May 2023 (Table 6). Our results showed that anxiety toward COVID-19 was the highest in 2021 and decreased continuously, reflecting the social situation during the three survey periods. When we conducted the first survey in May 2021, Japan was still in the third state of emergency, and people were asked to take precautions, such as staying home, working remotely (less than 30% of the workforce was in an office), and refraining from leisure activities. However, from 2022, these restrictions were eased, allowing a gradual return to pre-COVID-19 conditions. This shift allowed for increased FTF communication, which enabled the participants to receive more social support in 2022 and 2023 than in 2021, leading to higher levels of SWB in those years. However, depression tendency peaked in 2022, probably because the participants faced changes in their daily lives while switching from COVID conditions to “new normal” conditions (e.g., re-adapting to FTF classes). Indeed, the need to adjust one’s lifestyle to accommodate changes was identified in a previous study as a post-COVID mental health concern [39].

The following observations were made over time for the three patterns (Table 7): First, the scores for generalized trust were almost the same for the three patterns, and there were no significant differences between male and female students, except for Pattern 1 in 2021. These results imply that young adults’ levels of generalized trust did not change during the COVID-19 pandemic.

The results showed that young adults’ levels of anxiety toward COVID-19 decreased annually in all three patterns, whereas their depression tendency peaked in 2022 and decreased to its lowest level in 2023. All users received increasing social support annually. Therefore, SWB levels increased over the three consecutive years for all three patterns. However, as an increasing number of universities decided to return online classes to FTF, students might have felt stressed, which would have led them to have the highest level of depression tendency in 2022. After they became accustomed to more FTF classes and communication with others, their depression tendency decreased to its lowest level, and they received the most social support and had the highest levels of SWB in 2023. When looking at the differences among the three patterns, users in Pattern 3 tended to have the highest levels of depression tendency, with the least social support and the lowest levels of SWB. Users in Pattern 2 received the most social support, but those in Pattern 1 tended to have the highest SWB levels, except in 2021. This may be because the users in Patterns 1 and 2 had more intimate communication partners, whereas those in Pattern 3 tended to post and share common hobbies with others (Table 5). Furthermore, female students’ levels of anxiety toward COVID-19 were higher than those of male students in all three patterns, and their depression tendency was higher than that of male students in Pattern 1. These results support those of previous studies (e.g., [40]) and indicate that female students are more susceptible to emergencies such as COVID-19.

Both regressions for the main and interaction effects demonstrated that personality trait factors were more closely related to SWB than to Internet or social media use across these patterns. First, the results showed that among the three patterns over the three years, social support and depression tendency had strong positive and negative effects on SWB, respectively. These results confirm the implications of previous studies [17]. Therefore, it is suggested that receiving more social support from others and reducing depression tendency will help improve SWB, regardless of the period.

Regarding the four factors of self-consciousness and friendships, we anticipated that young adults with higher levels of self-establishment and self-independent would have less self-pluralism (different selves in different situations, which is a composite of different “selves”), which would help improve their SWB, whereas higher levels of self-indeterminate and self-variable factors would lead to greater self-pluralism and lower levels of SWB [41]. However, from the main effects, we observed positive relationships between self-indeterminate and self-establishment factors and SWB, and a weak negative relationship between the self-independent factor and SWB (see Table 8).

Traditionally, it has been believed that if a person fails to establish their identity during adolescence, they will suffer from an unclear sense of existence and a loss of direction later in life, which Erikson [42] referred to as “identity diffusion.” Additionally, Asano [43] indicated that social media use fosters young adults’ self-pluralism, which may not be ideal. However, our results demonstrated that it had positive effects on SWB improvement in all three patterns, which implicated that, for the digital generations, displaying different sides of themselves would help them maintain their mental health in this “Mobile x Social Era”, as the intimacy level of communication partners on different social media platforms differ.

Concerning the effects of the desire for self-presentation and admiration, we assumed that the four factors would have negative relationships with SWB since they are related to self-esteem. Higher levels of rejection avoidance, praise acquisition, self-appeal, and topic avoidance reflect a lower level of self-esteem and, thus, a lower level of SWB [44]. However, we found a positive relationship between the praise acquisition factor and SWB in Patterns 1 and 2 for both genders, which is contrary to the findings of prior research (e.g., [45]). According to Kitamura et al. [46], social reward motivation results in a decrease in the use of negative emotional words [16]. Thus, participants with a stronger desire to gain admiration had higher motivation for social rewards, leading to fewer negative expressions and improved SWB. Concurrently, the COVID-19 environment may have played a role as it reduced people’s FTF interactions, increasing their desire to gain praise via online communications compared to before COVID-19. Similarly, Deloitte [47] suggested that praising was an essential tool for engaging people during the pandemic. Therefore, our findings may reflect young adults’ desire to engage with others through social media during the pandemic. However, because the effects of praise acquisition were only found in Patterns 1 and 2, they may not be effective when connecting more unknown people via social media with higher levels of visual anonymity, such as Discord, if people use them to play games and share common hobbies.

According to our findings on the three social media use patterns studied, social media usage time and posting frequency did not relate to SWB as much as personality traits. There are several possible explanations for this. First, over 99% of the participants used LINE, indicating that this application has become an indispensable communication tool in the daily lives of university students. This may be the primary reason why 52.9% to 63.2% of participants posted on LINE almost every day and used it to reply to their friends in their daily lives (see Table 4 and Table 5). However, their usage time led to lower SWB. Kato and Kato [48] clarified that the more “friends” and “groups” registered on LINE, the more likely that users will experience negative emotions. Users who wait for replies to read messages are more likely to experience negative emotions than those who wait for unread messages, which is more pronounced in highly dependent groups [49]. Compared with Twitter, it is more difficult to “block” or “delete” friends on LINE as it connects users to individuals involved in their daily lives. Moreover, “LINE fatigue” occurs when users spend an excessive amount of time using LINE, which may negatively affect their mental health.

In addition, over 80% of the participants used Twitter, with the proportion of those posting almost daily ranging from 23.9% to 50.4%. As pointed out in previous studies (e.g., [14]), Twitter is used to share common hobbies unrelated to SWB. This trend was more obvious on Instagram, as only 10.9–24.4% posted on it almost every day, and most of the content consisted of photos, videos, and topics related to themselves and their friends. Although Discord was primarily used to reply to friends, it was also used to share common hobbies; this trend was more obvious for males. This demonstrates the unique use of Discord among Japanese adults. Furthermore, although TikTok has become increasingly popular among younger generations in Japan, over 80% did not post anything on it during the three-year survey period, which means that they might have used it only for viewing messages and videos; therefore, it had no relationship with their SWB. Finally, the main effects of Internet usage time via computers and tablets were only found in Pattern 3 in 2022 and 2023, respectively. In 2021, the interaction effects showed that female students’ Internet usage time on smartphones may have decreased and increased their SWB levels in Pattern 2 and Pattern 1, respectively, whereas male students’ Internet usage time had negative effects on their SWB in Pattern 1. These results support the implications of previous studies (e.g., [50]) and suggest that different devices may have had different effects on users when connected to the Internet according to gender, even during the COVID-19 pandemic. However, as the interaction effects for Internet usage duration and gender were small and not consecutive, further examination is required.

The results of our study support the cognitive bias model (e.g., [15]) in terms of generalized trust, self-indeterminate, self-establishment, rejection avoidance, and praise acquisition for at least two years or between the two patterns. We consider these as new implications when exploring the effects of the traditional cognitive bias model on mental health. However, the discontinuities of the personality traits indicate that even the same personality trait might have different effects on SWB given different social media combinations among genders, which is related to the social network mediation model. Except for the weak effects of Instagram usage time and posting frequency on LINE and Instagram, no significant effects on social media use were detected. These findings suggest the necessity of future examinations of the social network mediation model, with a focus on different social media use patterns.