8. The method of claim 1 , in which each vector represents a corresponding user in a community, and each feature of each vector represents the corresponding user's click-behavior with regard to a content item.
8. The method of claim 1 , in which each vector represents a corresponding user in a community, and each feature of each vector represents the corresponding user's click-behavior with regard to a content item. 9. The method of claim 8 , further comprising identifying a pair of vectors (x, y) as representing a group of colluding users.