PERSONALIZING CONTENT ITEMS

Methods, systems, and apparatus, including computer programs encoded on computer storage media, for personalizing content items. One of the methods includes identifying, by a computer system, a content item to be provided to a client device of a first user. The method includes determining that the content item is capable of including a contextually-related content item associated with a second user, the second user having a relationship of a first user in a social affinity graph. The method includes altering, by the computer system, the content item to include the contextually-related content item. The method includes providing the content item to the client device.