Yep, it's a rumour - but, quite frankly, we'd be utterly astonished if there wasn't some truth to this here one. iMore has done a little delving around into the Beta 3 of iOS 5.1 and reckons Facebook is present in the contacts app - right alongside good old Twitter.
Of course, tweetage was something the Cupertino clan finally natively bundled into the last major iOS release meaning Apple users could do the basics like tweet directly from their camera app, rather than having to go around the houses to attach a pic.
In fact, the only reason Facebook wasn't there - to the shock and mirth of power-users - was because Apple and Facebook were unable to reach an agreement in time.
That could still be the case - with iMore publishing this with a health warning: there is the chance that this is not some big exclusive, but merely what's called "legacy code."
If Facebook is fully integrated into the OS rather than just being a third party solution, it would surely make uploading photos and updates, tagging people and just using it in general a much more streamlined and elegant experience.
Guess we'll just have to sit back and wait on this one..