The main aim of the study was to investigate Instagram’s impact on users’ mental well-being and behavior (including purchasing activity) during the pandemic. The study was conducted on 359 respondents who stated that they used Instagram regularly. Three questionnaire tools were implemented: the Instagram Usage Questionnaire (original tool), the MSEI Multidimensional Self-Esteem Inventory by O’Brien and Epstein, and the Personal Data Sheet (original tool). Significant negative correlations were obtained between self-esteem components: self-control, competence, lovability, identity integration, defensive self-enhancement, frequency, and intensity of social media use. The respondents declared no negative influence of Instagram on their current mental well-being. One group (21.2%) considered the influence beneficial to their mental well-being. Observing others’ Instagram profiles did not influence the activity of (most) users, including changing appearance, lifestyle, or making purchase decisions. Moreover, most respondents declared not making purchase decisions based on Instagram ads. The analysis of differences between purchasing and nonpurchasing groups showed significant discrepancies in these self-esteem components: likability, personal power, and competence. Significant differences regarding physical attractiveness, overall self-esteem, lovability, likability, personal power, competence, and vitality were reported between men who made purchase decisions based on Instagram messages and those who did not. Among women, these differences concerned only defensive self-enhancement. The results confirm that prolonged/frequent use of social media can have a negative impact on self-esteem (including identity integration and self-control). However, a positive impact of Instagram on mental wellbeing was observed. Following (mostly) acquaintances (not influencers) and not undertaking activity (regarding purchasing decisions of Instagram-advertised products) may indicate emerging changes in society due to the pandemic.