17. A method comprising: viewing, by a user via a computer system, first content; obtaining, by the computer system, a first vector containing numerical values quantifying the affinity of the first content for each subject matter domain of a plurality of subject matter domains; providing an online advertisement linked to second content; obtaining, by the computer system, a second vector containing numerical values quantifying the affinity of the second content for each subject matter domain of the plurality of subject matter domains; obtaining, by the computer system, a user vector containing numerical values quantifying the affinity of the user for each subject matter domain of the plurality of subject matter domains, the user vector being derived as least in part from the first vector; selecting, by the user, the online advertisement; directing, by the computer system in direct response to the selecting, the user to the second content; and calculating, by the computer system, a match value quantifying the similarity between the second vector and the user vector.
18. The method of claim 17 , wherein the user vector is derived at least in part from a third vector corresponding to a third content viewed by the user via the computer system, the third vector containing numerical values quantifying the affinity of the third content for each subject matter domain of the plurality of subject matter domains.