It’s not actually the functionality that works for Facebook, it’s the fact that people have a Facebook account that they check. As more and more people quit, it becomes more and more of a bother to continue using Facebook to organise things, but when they have 3 billion users, they’ll probably remain a good place for groups and events for quite a long time to come, unless someone figures out how to do it with e-mail, which is basically the only other thing that everyone also has.
As others point out, social media sort of leaves a gap as far as being “the people registrar” or a modern form of the yellow pages. But as long as so many of us remain on there because it is what it is, then they can really continue to fuck around quite a bit before “dying”. I think it’ll be legislation that kills them in the end, because unlike other large American tech companies, Facebook doesn’t seem to have a plan that doesn’t involve making money off privacy data. Even though Google is obviously struggling with being an advertisement company first, they could potentially shift to other products, but what can Facebook really do?