As well you shouldn't. Back at the beginning of the year the NYT dropped a bunch of leaked memos about such a feature and at the time Facebook claimed that they weren't planning to release such a thing.

Now there's actual code for it, and their newest leaked memos prove they actually were planning to release such a thing and even had planned to do so at a time when people would be distracted by politics or other hot button issues so they could fly under the radar and not face as much backlash.