Friend of a friend (FOAF) is a phrase used to refer to someone that one does not know well, literally, a friend of a friend.
In some social sciences, the phrase is used as a half-joking shorthand for the fact that much of the information on which people act comes from distant sources (as in "It happened to a friend of a friend of mine") and cannot be confirmed. It is probably best known from urban legend studies, where it was popularized by Jan Harold Brunvand.
The term was apparently first published by Rodney Dale in his 1978 book The Tumour in the Whale, in which he discussed the "FOAFtale".
The rise of social network services has led to increased use of this term.