1. A method of selecting content items for presentation to a user, 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.