My main point is that the lack of Facebook affects the process by which these real friendships form. In a context where Facebook is used by most of your peers,

My main point is that the lack of Facebook affects the process by which these real friendships form. In a context where Facebook is used by most of your peers, the lack of it creates an additional barrier to socialization that has subtle negative effects which compound over time. The result is that you can miss out on forming real friendships because the cementing period is stunted by the reduced amount of unplanned interactions. Of course, this is highly ironic when seeing that strong friendships are resilient to the absence of social media or the passage of time. The concept may seem strange but it's actually no different than someone growing more distant from a group of friends because they habitually miss out on the weekly Quiz Night or something. It's not so much that people force you to communicate by one means, but rather that the friction involved reduces the amount of interaction in an automatic way. As kinkrtyavimoodh pointed out, it's an organic process and not really a conscious choice by people to isolate others.