6. The method of claim 1 , further comprising: determining that the particular resource identifier is stored for the first user in the bookmark database; determining that the particular resource identifier is stored for the second user in the bookmark database; and in response to determining that the particular resource identifier is stored for the first user in the bookmark database and determining that the particular resource identifier is stored for the second user in the bookmark database, identifying that the first user and the second user belong to a particular cluster of users.
6. The method of claim 1 , further comprising: determining that the particular resource identifier is stored for the first user in the bookmark database; determining that the particular resource identifier is stored for the second user in the bookmark database; and in response to determining that the particular resource identifier is stored for the first user in the bookmark database and determining that the particular resource identifier is stored for the second user in the bookmark database, identifying that the first user and the second user belong to a particular cluster of users. 7. The method of claim 6 , further comprising: in response to identifying that the first user and the second user share a resource identifier, exposing the second user to one or more additional resource identifiers identified by the first user.