12. A computer system for developing a user profile of a target user, the system comprising a non-transitory computer readable medium storing instructions for: determining associated topics for each of a plurality of digital content items; retrieving access data indicating interactions of the target user with at least one of the digital content items; selecting profile topics for the target user based on the topics associated with the digital content items and the retrieved access data; storing the selected profile topics in association with the user profile of the target user; identifying, from a collection of other user profiles, candidate topics co-occurring with a target profile topic from the user profile of the target user; calculating, for each candidate topic, a count at which the candidate topic co-occurs with the target profile topic across the collection of user profiles; ranking each candidate topic based on the count at which the candidate topic co-occurs with the target profile topic; and selecting an additional topic to include in the user profile of the target user from the candidate topics based on the ranked candidate topics.
12. A computer system for developing a user profile of a target user, the system comprising a non-transitory computer readable medium storing instructions for: determining associated topics for each of a plurality of digital content items; retrieving access data indicating interactions of the target user with at least one of the digital content items; selecting profile topics for the target user based on the topics associated with the digital content items and the retrieved access data; storing the selected profile topics in association with the user profile of the target user; identifying, from a collection of other user profiles, candidate topics co-occurring with a target profile topic from the user profile of the target user; calculating, for each candidate topic, a count at which the candidate topic co-occurs with the target profile topic across the collection of user profiles; ranking each candidate topic based on the count at which the candidate topic co-occurs with the target profile topic; and selecting an additional topic to include in the user profile of the target user from the candidate topics based on the ranked candidate topics. 15. The computer system of claim 12 , wherein the determined associated topics each has a usefulness weight indicating how useful a topic is in representing its association with the digital content item, and the instructions further comprising: selecting the profile topics based on usefulness weights.