There are actually eight different factors which the courts use as a "test" for infringement. Of those eight, I would say a majority fail the infringement test for Facebbook. Mainly because "FB" is not part of its primary trademark. Thus, the "FB" presents a weak case for the "Strength of the plaintiff's mark." element.

Here are those 8 factors for argument sake:

http://www.bitlaw.com/trademark/infringe.html#elements