It’s indeed a custom font created by a designer called Joe Kral
(‘Klavika Bold’ is accepted as the closest font you can get):
Edited on Jan 29, 2013 at 01:41 by rocamaco
It is a custom font, indeed. Klavika is close, but the font Facebook Letter Faces is based on the logo, so I think that is the best one.
All times are CEST. The time is now 16:48