Facebook aside you really see this across the board on almost any platform, that once the product reaches it's 1.0 stage, (where it is good, does what the users want it to do, and has realised its vision) it begins a process of gradual decay, as the focus of the product managers (now panicking to find some statistics to improve to show their bosses) shifts from "building functionality" to "increasing engagement/reten…
In short, see 80/20 rule. Once you've hit 80 you probably should look at growing horizontally. I don't use Facebook but I'm under the impression they've done this somewhat with local market places, games & video streaming. It sounds to me that you're sick of common abused marketing tactics or maybe it's just that I am & therefore that's how I read your post. Gamification, Deadly Sins, Reciprocity, etc..
To many it happens after hitting 40.