Last.fm is a music-based social network with over 21 million users. It tracks users' listening habits through its "audioscrobbling" technology and provides recommendations and charts of popular artists and songs. Last.fm generates revenue through advertising and allows users to stream full songs up to three times. Though still young, Last.fm has been successful in attracting users daily, was sold for $280 million, and signed deals with major record labels. Its main challenge is ensuring advertisers remain engaged.