27. The non-transitory computer readable storage medium of claim 25 , wherein the CAPTCHA manager module is further configured to: select a type of information used to form the challenge question based on a measure of affinity between the user and the other user.
25. A non-transitory computer-readable storage medium storing computer-executable code comprising: a member profile store configured to store user profiles for a plurality of users of a social networking system; a connection store configured to store a plurality of connections between the users of the social networking system; a web server module configured to: receive a request from a requestor for access to a computing resource, wherein the request is associated with a user of the social networking system; a CAPTCHA manager module configured to: select, for forming a challenge question, another user of the social networking system from a plurality of other users connected to the user via the social networking system, the selection based on a rate of interactions between the user and the other user; obtain information from a user profile of the other user; determine whether the request is received from a suspicious source; determine difficulty level of the challenge question based on whether the source is suspicious; and form the challenge question according to the determined difficulty level, wherein an answer to the challenge question comprises information obtained from the user profile of the other user; and the web server module further configured to: send the challenge question to the requestor; and responsive to receiving a correct answer to the challenge question, allow the requested access.