It's not. The update is legit, and from Facebook. They are pushing out an update outside of the Play Store (which is a direct violation of the developer agreement, by the way) to some users to test new features. If you have the setting checked to allow installations from unknown sources (sideloading) you might have seen this. You also might still be stuck fiddling with the notification that won't go away. To fix that, hook up with some Wifi -- Facebook won't download the update while you're using your data connection.
Let's ignore that Facebook may be breaching their developer agreement. Lets also ignore that they are silently pushing software to a users phone, when that user is unaware and didn't ask for it. We'll also overlook the way Facebook thinks all Wifi is free and unlimited. But we can't overlook the fact that they let this go on for three days and still haven't posted a little something to their blog or dropped a little blurb in the Google Play app description to let the users -- you know, the ones who are worried what the hell this fake Facebook download is doing -- know what was going on.
[via Android Central]