1. A method comprising: receiving from a publisher, by a processing device of a server, metadata for customizing presentation of a social feature with a content item provided by the publisher, the metadata describing a graphic, and an input associated with the social feature to be provided by a user of a custom media player executing on a client device; transmitting, to the custom media player one or more instructions that cause the custom media player to present with the content item, in accordance with the metadata, a user interface element comprising the graphic and enabling the user of the custom media player to provide the input associated with the social feature; receiving, by the processing device, event data associated with the presentation of the social feature by the custom media player, the event data comprising the input associated with the social feature provided by the user of the custom media player; generating a report based on the event data comprising the input associated with the social feature provided by the user of the custom media player, the report describing user interaction with the social feature in the custom media player; and providing the report to the publisher.