1. A method comprising: training, by at least one computing device, a machine-modeled kernel jointly modeling content, semantic information and social network information, the kernel for use with a digital content annotation and recommendation system, the training comprising building the kernel using a content kernel trained using a training set comprising content feature information, a semantic kernel trained using semantic feature information of the training data set and a social network kernel trained using social network feature information of the training data set, an implicit social network determined from a plurality of content items is used in determining the social network feature information used in training the kernel; identifying, by the at least one computing device, one or more annotations for a test content item of the content data store other than the plurality of content items used to train the kernel, the identifying using the trained kernel and the test content item's content feature information to identify the one or more annotations; and serving, by the at least one computing device and to a client computing device of a user via an electronic communications network, the test content item based on the one or more identified annotations and an interest of the user, the serving of the test content item to the user computing device resulting in the test content item being output by the user computing device.