1. A method comprising: training, by at least one computing device, a kernel jointly modeling content, semantic and social network information of a training set comprising a plurality of content items, each content item of the plurality having associated content, semantic and social network feature information used in training the kernel; and identifying, by the at least one computing device, one or more annotations for at least one test content item other than the plurality of content items used to train the kernel, the identifying using the trained kernel and the content item's content feature information to identify the one or more annotations.