I've noticed that quite a lot of my friends are now using it pretty much exclusively for a) sharing photo's (because that's where everyone is to share them with

I've noticed that quite a lot of my friends are now using it pretty much exclusively for a) sharing photo's (because that's where everyone is to share them with) and b) as an interactive contacts database. Even game usage seems to have plummeted on my feed. I think you are right; Facebook has a lifecycle - so it's reach will probably flatten out (I'm guessing it will drop a bit first) and it will end up with a slow churn of new users taking the place of inactive ones.