Adolescents commonly use smartphone social media apps, which can affect their perceived social support (PSS). However, study results on social media’s effect on PSS differ, because they employ a self-reported time in social media use and concentrate only on between-person differences. They often neglect the social-anxiety level, which might be important. Our study investigated whether the within-person day-to-day changes in the time spent in two types of social media apps (communication and social networking) influence the daily PSS and vice versa, and whether social anxiety moderates these relationships. Using a mobile application that was installed on adolescents’ smartphones (N = 194), we collected the daily objective time spent in apps and the self-reported PSS for 14 days. Social anxiety was assessed with a baseline survey. The models showed that day-to-day changes in communication and social networking app use did not influence the PSS the next day and vice versa. Social anxiety did not moderate these relationships. We further elaborate on the variability related to the within-person effects.