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 acronym FOAF was coined by Rodney Dale and used in his 1978 book The Tumour in the Whale: A Collection of Modern Myths.
The rise of social network services has led to increased use of this term. Six degrees of separation is a related theory.
Bo Anderson made an analysis of the friend-of-a-friend relationship in connection with his criticism of balance theory.
We have all encountered cases in which somebody has said, "You should meet so-and-so", only to find that we have little in common with that person, even though he or she was introduced to us by a mutual friend...In some friendships the persons value the exclusiveness of their relationship and are therefore not likely to let others into it. Friends differ from acquaintances in that they are not merely slots in a grid of social network relationships, but are valued for their personal, unique qualities. Hence, when I relate to a friend of a friend, I need to know something about the perceptions and exchanges that make up this friendship. My reaction to my friend's friend (or spouse) may even be unfavorable, although I may also well understand and sympathize with my friend’s affection for her, given his needs, perceptions, interests and so on.