Could it be that the license to process the volume facebook has in mind is near or higher than the price of the company? I don't know the price model for the face API but it's not unreasonable to consider a tipping point.

Probably not. It's so far been a symbiotic relation - face.com needed facebook as much as facebook needed face.com.

Facebook is probably making sure that this symbiotic relation is not going to be threatened by a competitor buying face.com