Hmm... just thinking out loud here...
I guess history tells us that these sorts of sites do lose popularity when some other fad comes along. That's probably why he bought out Instagram. The general populous moves from one fad to another. I think the uses of Facebook are changing more towards groups, the sharing of information and keeping in contact with old friends as people are getting disenchanted with the sharing their life side of it. I know many use messenger more than their Facebook page. However, restrictions and censorship could also drive away that sharing side of things.
I'm with a group who are all about birds and poultry, so sales are often made as people look for different breeds. Facebook has banned the sales of animals though, making it harder to advertise. Often you have to be careful how you word a sale or make it a discussion instead. How long before these pages are shut down though?

Okay, you've convinced me that facebook can't last in the format it has now. It would need to evolve. However, he still has a bit longer to render Instagram unbearable for the general populous.