You're talking about two different things here (and I'm not condoning either, to be clear.)
1) UI = a clearly documented way to configure all features and make the software work exactly how you want.
2) UI = load a web page and try to do the thing you wanted to do (in this case communicate with some specific people).
FB is clearly terrible at 1 but pretty alright at 2.
it is a UI designed to be hard to use