Does social media alleviate or exacerbate loneliness? Past research has shown mixed results regarding the relationship between social media usage and loneliness among younger and older adults. Unlike younger individuals, older adults may decrease their loneliness through social media interactions. Additionally, previous research has indicated that the link between social media use and loneliness can vary depending on one’s shy tendency. Therefore, this study aims to explore the relationship between individuals’ social media use and loneliness while considering age and shyness tendency as moderating variables. The study employed a questionnaire survey conducted through convenience sampling, resulting in 234 valid responses from participants in Northern Taiwan. Among them, 113 were college students (aged 18 to 25, average age 19.40), and 121 were older adults (aged 50 to 82, average age 60.81). Using hierarchical regression analysis, results indicated that (1) age moderates the relationship between personal social media use and loneliness. Minimal differences were observed among younger individuals, but among older adults, increased social media usage time was associated with a significant reduction in loneliness. (2) Shyness tendency moderate the relationship between personal social media use and loneliness. Individuals with higher shyness tendency experience an increase in loneliness as their social media usage time lengthens.