As social interactions are increasingly taking place in the digital environment, online friendship and its effects on various life outcomes from health to happiness attract growing research attention. In most studies, online ties are treated as representing a single type of relationship. However, our online friendship networks are not homogeneous and could include close connections, e.g. a partner, as well as people we have never met in person. In this paper, we investigate the potentially differential effects of online friendship ties on mental health. Using data from a Russian panel study (N = 4,400), we find that - consistently with previous research - the number of online friends correlates with depression symptoms. However, this is true only for networks that do not exceed Dunbar's number in size (N <= 150) and only for core but not peripheral nodes of a friendship network. The findings suggest that online friendship could encode different types of social relationships that should be treated separately while investigating the association between online social integration and life outcomes, in particular well-being or mental health.