Instagram’s unidirectional following model favors fans over known friends.

Evidence

Following on Instagram is unidirectional; unlike Facebook, a user may follow another without permission, and, like Twitter, there is generally no mutual expectation of following. This model creates an environment more conducive to fans or curious strangers than to known “friends,” as users can blithely add any account that looks interesting to their stream. — Alice E. Marwick, Instafame, 23