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…
IMO modern mainstream operating systems are also a good example of this. An operating system has basically three jobs: abstract the hardware from application software, provide libraries for commonly used functions, like GUIs, and provide the user with some kind of environment/shell to run their applications in. But somehow, this shitshow of an industry has converged on needing to release a major update every year. Imagine being forced to come up with new features for, and release a major update of, a feature-complete product every time the planet completes a lap around its star just because the marketing department exists and demands that it so happens because reasons. Oh, and what if you run out of feature ideas? Then of course you redesign the UI! UI redesigns never get old.