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Facebook Letter Faces  Sugerida por Wolf Lambert 

It’s indeed a custom font created by a designer called Joe Kral.

(‘Klavika Bold’ is accepted as the closest font you can get):

http://www.myfonts.com/fonts/processtype/klavika/bold/

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.