Rdio is a digital music service that provides on-demand access to over 20 million songs, social networking features, and music recommendations. Founded in 2010 by Janus Friis and Niklas Zennström, Rdio has raised $17.5 million but is not yet profitable, with 120 employees. While offering strong recommendations and integration features, Rdio likely won't overcome the lead of competitor Spotify in the digital music streaming market.