[sarcasmic]

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.

[sonnyblarney]

A critical mass of individuals in most people's lives, and a critical mass of activities and group pages are on FB, to the point wherein most won't give it up for that reason alone; "I'd miss out on old friends or not be able to contact them, also, my kid's school and local playhouse are on there" - is a common refrain.

In those terms - there is no substitute for FB.

As far as your view of how people view relationships - the reason I don't buy it, is because of the simple fact: most people are not quitting FB. Keeping their account costs little, and enables those 'weak' relationships. So they will stay. And FB has the critical mass.

There is no substitute, at least in 2018. Really the only choice is turning it off which I think will happen, gradually.