Facebook is is a trademark. Not only that, but it is a trademark of one of the most powerful companies in the world today. Probably only second to Google.
You don't use trademarked names in a domain unless you want to be dragged through the coals.
If you registered this domain in the last 5 days or less, ask your registrar to grace delete it. Otherwise, just tell them to drop it (you lose all the money, but that is the price you pay for not researching what and what not to register before you actually register it).
But let's say it did not a trademark problem (which it does), it's still a bad domain.
What people do when they want to register a domain that pertains to facebook, they just use FB. I'm not saying that this is legally sound, but a lot of FB-related products/services do seem to do that without problems (seemingly anyhow).