Disclosed are various embodiments for a social networking behavior-based identity system that employs social networking data that a user has elected to share through an opt-in procedure. An assertion of a user identity is received from a client. It is determined whether the assertion of the user identity specifies a correct security credential. Social networking data identifying a circle of friends is received. It is determined whether the user identity belongs to a user at the client based at least in part on a reputation of one or more members of the circle of friends and whether the assertion of the user identity specifies the correct security credential.