We might be at a local maximum, but we're nowhere near the peak. What we're seeing is an end of the race to the bottom. What's happened is that most of the major social networks only hire and promote people who make decisions primarily based on analytics. As a result these sites no longer have any real communities or organic content, and have been completely hollowed out by math-friendly refrigerator magnet junkfood.…
Anecdotally, I know several people who say groups are the only thing keeping them on the network.
Interestingly, the groups also dovetail well into the continued growth of their ad platform; they tend to be interest based, so they help with ad targeting.