Many people can and do go elsewhere, because an individual can have presence on multiple platforms. And while one's social graph in different platforms looks different and is missing some people they know, the network is both self-healing and self-fulfilling, in that it can be reasonably assumed that strong connections can survive irrespective of the network, while weak connections may not, and all connections that didn't survive will be retroactively redefined in one's mind as weak.

It's not like people genuinely use Facebook to discover brand new individuals (as opposed to bands or groups), whereas they'd frequently do that on Instagram, Twitter, or Tumblr. And for interests and activities like music discovery, live music, celebrity fandom, neighborhood activities, and classifieds, other viable alternatives do exist, even if they aren't packaged alongside a rolodex of all the people you went to college with, or your vacation photos, or some isometric farming simulator you used to play 8 years ago.