3. The method of claim 1 , wherein calculating semantic similarities between the content items comprises calculating cosine similarities between topic vectors of the content items, the topic vectors of the content items each including a list of topics, each topic including a semantic descriptor of one or more of the content items.
3. The method of claim 1 , wherein calculating semantic similarities between the content items comprises calculating cosine similarities between topic vectors of the content items, the topic vectors of the content items each including a list of topics, each topic including a semantic descriptor of one or more of the content items. 4. The method of claim 3 , where calculating semantic similarities between the content items comprises calculating semantic similarities between segments, including calculating cosine similarities between the topic vectors of the respective segments, where the topic vector of each of the segments is calculated using an average of topic vectors of content items associated with user actions clustered into the segment.