16. A computer implemented method comprising: maintaining user profiles for a plurality of users of a social networking system; maintaining a plurality of connections between the users of the social networking system; receiving a request from a requestor for generating a challenge question based on information related to a user of the social networking system, wherein the requestor receives a request for access from the user; selecting, 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 on interactions between the user and the other user; obtaining information from a user profile of the other user; determining whether the request is received from a suspicious source; determining difficulty level of the challenge question based on whether the source is suspicious; forming the challenge question according to the determined difficulty level, by a processor, wherein an answer to the challenge question comprises information obtained from the user profile of the other user; and sending the challenge question to the requestor wherein the requestor allows a requested access to the user responsive to receiving a correct answer to the challenge question.
16. A computer implemented method comprising: maintaining user profiles for a plurality of users of a social networking system; maintaining a plurality of connections between the users of the social networking system; receiving a request from a requestor for generating a challenge question based on information related to a user of the social networking system, wherein the requestor receives a request for access from the user; selecting, 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 on interactions between the user and the other user; obtaining information from a user profile of the other user; determining whether the request is received from a suspicious source; determining difficulty level of the challenge question based on whether the source is suspicious; forming the challenge question according to the determined difficulty level, by a processor, wherein an answer to the challenge question comprises information obtained from the user profile of the other user; and sending the challenge question to the requestor wherein the requestor allows a requested access to the user responsive to receiving a correct answer to the challenge question. 17. The method of claim 16 , further comprising: receiving a suspiciousness score value from the requestor; and determining the difficulty of the challenge question based on the suspiciousness score value.