That's 1/3 of the people on the planet. Even if you don't trust that number, its hard to criticize them because they don't have enough users.
In the US & Canada, the number of active accounts has been in decline.
https://techcrunch-com.cdn.ampproject.org/c/s/techcrunch.com...
Granted, only by ~1 million, but that's not growth.
Facebook will always be profitable in some respect, just as AOL is still profitable, but it's entirely possible that people are going to get bored and move on to other things. Old folks will still use Facebook to debate about which Denny's in town has the better supreme skillet, but there's no sign that Facebook will hold on to the ubiquity it currently has.
Maybe Facebook will continue to mean something to the world outside the west, which a graph in the story I linked to seems to suggest.