1. A computer-implemented method comprising: generating a first global score for a content item obtained from heterogeneous data sources, the first global score being independent from users and indicating popularity or importance of the content item within a source stream that produced the content item; generating a second global score by normalizing the first global score across a plurality of source streams; determining a first candidate user for the content item based on the second global score and by querying a social graph using metadata associated with the content item; computing, based on a model, a first user score for a pair of the first candidate user and the content item to evaluate a probability that the content item appears in a stream of content of the first candidate user; determining whether a threshold is satisfied based on a volume and a quality of other content items on a same topic as the content item; and determining whether to present the content item in the stream of content of the first candidate user based on the first user score and whether the threshold is satisfied.