Acronym for "too hot for facebook".
Said by those wishing to comment on revealing, sexually suggestive, or just plain sexy facebook pictures that they're friends have posted.
Also used as an adjective to refer to photos that are not necessarily on facebook (although likely will be in the near future).
By the way, facebook is the Devil.
facebook photo comment: I think I can see your nipple in this one!! thffb!
casual online/text conversation: Hittin the beach this Saturday :D gonna all kinds of thffb pics!