So you go through Tor to access Facebook, where you immediately have to log in, and...
You really don't know anybody who uses Facebook pseudononymously? If you make an account called 'Hootie McBoob' you might get dinged, but there are thousands of 'Bill Riker's (have some fun with it).
If you're coming in from your home IP or a Verizon or AT&T mobile, you're gonna be decloaked in a hurry, even by a passive listener. So, if you want to participate in a community that's on Facebook but not be known to the outsiders, Tor makes sense. Right now you can exit Tor on one of the spooks' exit nodes, but then you're just enabling the traffic analysis. By offering Tor directly, you avoid the risk of using an additional hostile exit node.
This looks to be Facebook engineers doing the best they can given the cards they're holding. It's obviously more secure to not use any social networking systems at all, but if you rank security/privacy below functionality for some uses, this move makes sense to improve the situation.