1. A method, comprising: reducing, by a system comprising a processing system including a processor, a sampling size of a population of social network users of an on-line social network based on a comparison of seed information to a population of the social network users to obtain a reduced population of social network users of the on-line social network; sampling, by the processing system, the population of the social network users to obtain first sampled social network users; sampling, by the processing system, the reduced population of the social network users utilizing a crawl algorithm to obtain second sampled social network users; generating, by the processing system, a social network graph based on the first sampled social network users and the second sampled social network users, wherein the social network graph comprises an arrangement of the first sampled social network users and the second sampled social network users based on relationships between members of the first sampled social network users and the second sampled social network users arising in the on-line social network; characterizing, by the processing system, a cluster of social network users within the reduced sampling of the social network users; and identifying, by the processing system, an outlier in the reduced sampling of the social network users based on the characterizing of the cluster of social network users, wherein the outlier does not conform to the social network graph, wherein the second sampled social network users comprise current users and users being followed by the current users, wherein the second sampled social network users comprise inactive users of the on-line social network, wherein the identifying of the outlier is based on collateral information combined with an intra-graph property of the social network graph, wherein the seed information further comprises information associated with a social network user of the social network users having a relation to a party who is not a social network user, and wherein the crawl algorithm comprises a randomized-walk algorithm with backtracking.