8. A computer program product comprising a non-transitory computer useable medium including a computer readable program, wherein the computer readable program when executed on a computer causes the computer to: generate 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; generate a second global score by normalizing the first global score across a plurality of score streams; determine 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; compute, 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 shows in a stream of content of the first candidate user; determine 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 determine 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.
14. The computer program product of claim 8 , wherein the computer readable program when executed on the computer also causes the computer to generate an expandable explanation for the content item in the stream of content of the first candidate user.