Facebook isn't going anywhere for a long time. They'll retain the privacy-clueless fossils with FB. They'll keep the aspirational set with IG. Anything even remotely competitive or alternative they'll just throw cash at and snap up.
eg. like having a myspace, or a .hotmail, or a @aol email account
Social networks have to be both useful and trendy in order to fully succeed....
messenger and whatsup are both very useful, FB itself is less, and it needs to be somewhat trendy in order to survive
Personally I find fb very useful to keep up with family and friends, but that might be not be the same for other people, especially very young folks