Christ people, it's just a damn trademark. Trademarks are in many regards the weakest form of IP protection available.
For starters, trademarks are severely limited in scope - they don't grant you sole ownership, they apply only in the realm of commerce, and especially in the case of trademarks that are themselves pre-existing words or phrases, have only limited applicability there. As an example, Activision Blizzard has a trademark on the word "Diablo" (Spanish for devil) - but if I were to sell a game called "Diablo Canyon Meltdown" (about a theoretical nuclear accident), they would not be able to stop me, nor would the be able to stop me from writing books, songs, or movies titled "Diablo" (unless I suggested some kind of direct connection to the game franchise).
Trademarks have other limitations as well. For example, even though trademarks are theoretically eternal, a trademark can become "genericized" - i.e. synonymous with the product or service in general, rather than a specific brand - and thus unenforceable. Examples of genericized trademarks include thermos and zipper.
TL;DR version - Facebook doesn't own the word "face" - they only own the right to use the word "face" in certain commercial contexts.
