Niklas Zennstrom Skype

Rdio, the social media music service backed by the founders of KaZaa and Skype, has raised another $17.5 million in its latest round of funding.

Its latest backer is Mangrove Capital Partners, who inadvertently broke this story over the weekend via Twitter. The start-up’s earlier funding was provided by Rdio co-founder Janus Friis through his investment entities as well as Atomico  and Skype. Atomico is the VC firm from Niklas Zennstrom (pictured) who was Janus’ partner in Skype and KaZaa, the music file sharing service. Currently available only in US North America., it’s going to use the new funds to expand its footprint to new platforms and countries this year.

Rdio’s service allows you to automatically share the music you’re listening to and discover new music through people you follow. It’s kind of what iTunes and Ping are trying to do but few think they’ve achieved.

Also interesting from Rdio is the fact they’ve added Warner Bros Records Chairman Rob Cavallo to its board. Cavallo, who only joined Warner Music in 2009, is best known as a producer of acts like Green Day, Dave Matthews Band and Alanis Morissette.

You might think that with all the troubles in the music industry investors might be staying away. Not according to Digital Music News, which estimates here that $33.8 millionhas already been committed to music-based start-ups this year including Rdio.