1. A computer-implemented method for facilitating online advertising, comprising: generating a semantic user profile for each of a plurality of users, based at least on user behavior data collected for each user; indexing the semantic user profiles; receiving at least one semantic query; performing a search over the indexed semantic user profiles based on the at least one semantic query, to enable selection of a target user segment from the plurality of users; determining a set of the indexed semantic user profiles resulting from the search; ranking the set of the indexed semantic user profiles based at least on an index of each of the indexed semantic user profiles of the set; and providing the ranked set of the indexed semantic user profiles, to enable the selection of the target user segment.