11. The system of claim 10 , the content item browsing history of the user comprising a short term browsing history identifying a content item most recently accessed by the user and a long term browsing history identifying one or more content items accessed by the user within a predefined time window, the system further comprising instructions that when executed by the at least one processor cause the at least one processor to: determine each topic's relevance by: adding, for each of the one or more content items identified in the long term browsing history, a weighted relevance of the topic to the content item to an aggregate relevance of the topic, the weighted relevance being determined using the topic's relevance to the content item and a weighting that is based on a time associated with the content item; combining the topic's aggregate relevance with a relevance of the topic to the content item identified in the short term browsing history to form a combined relevance for the topic.
11. The system of claim 10 , the content item browsing history of the user comprising a short term browsing history identifying a content item most recently accessed by the user and a long term browsing history identifying one or more content items accessed by the user within a predefined time window, the system further comprising instructions that when executed by the at least one processor cause the at least one processor to: determine each topic's relevance by: adding, for each of the one or more content items identified in the long term browsing history, a weighted relevance of the topic to the content item to an aggregate relevance of the topic, the weighted relevance being determined using the topic's relevance to the content item and a weighting that is based on a time associated with the content item; combining the topic's aggregate relevance with a relevance of the topic to the content item identified in the short term browsing history to form a combined relevance for the topic. 12. The system of claim 11 , further comprising instructions that when executed by the at least one processor cause the at least one processor to: determine, for each topic having a combined relevance, whether the user's interest in the topic is significant by comparing the topic's combined relevance with the user-interest threshold; and for each topic determined to be of significant interest to the user, use the topic to perform a per interest retrieval, the per interest retrieval retrieving content items in the plurality of content items that have a relationship with the topic; group any remaining topics, and perform a group interest retrieval to retrieve content items in the plurality of content items, each content item retrieved via the group interest retrieval having a relationship with all of the grouped topics.