1. A computer-implemented method for developing a user profile of a target user, the method comprising: 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, by a computer processor, 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.
3. The computer-implemented method of claim 1 , wherein the determined associated topics each has a topic strength indicating a degree of association of the topic with the digital content item, and the method further comprising: selecting the profile topics based on topic strengths.