11. A machine readable non-transitory storage medium storing executable program instructions which when executed by a data processing system cause the system to perform a method of selecting content items for presentation to a user, the method comprising: obtaining a list of candidate content items; obtaining metadata tags associated with the candidate content items; selecting at least one of the candidate content items for presentation to a user, based on previously stored user exposure scores for one or more metadata tags associated with the candidate content item, wherein the metadata tags refer to non-skipped portions of content items associated with the metadata tags and not to a skipped portion of the content items, the user exposure scores based on a number of occurrences of viewings of the non-skipped portions of the content items associated with the metadata tags, wherein the non-skipped portions of the content items are defined by demarcation points with respect to one or more skipped portions of the content items, each demarcation point being designated automatically in response to skipping within the content items based on user input during presentation of the content items, the designated demarcation points between the skipped portions and the non-skipped portions specifying a start point and an end point for the skipped portion.
11. A machine readable non-transitory storage medium storing executable program instructions which when executed by a data processing system cause the system to perform a method of selecting content items for presentation to a user, the method comprising: obtaining a list of candidate content items; obtaining metadata tags associated with the candidate content items; selecting at least one of the candidate content items for presentation to a user, based on previously stored user exposure scores for one or more metadata tags associated with the candidate content item, wherein the metadata tags refer to non-skipped portions of content items associated with the metadata tags and not to a skipped portion of the content items, the user exposure scores based on a number of occurrences of viewings of the non-skipped portions of the content items associated with the metadata tags, wherein the non-skipped portions of the content items are defined by demarcation points with respect to one or more skipped portions of the content items, each demarcation point being designated automatically in response to skipping within the content items based on user input during presentation of the content items, the designated demarcation points between the skipped portions and the non-skipped portions specifying a start point and an end point for the skipped portion. 14. The medium of claim 11 , wherein selecting at least one of the candidate content items for presentation to the user comprises: performing a Bayesian filtering analysis using the user exposure scores for metadata tags associated with the candidate content items.