Netherlands-based Nimbuzz has raised $15 million in a second round led by Naspers/MIH, with Nimbuzz's other major existing investor Mangrove Capital Partners also participating. It's already had $10 million from Mangrove (the original Skype investor). Apparently deals with 10 major social networks and three operators are already on the table. The latter see these kinds of apps as a way of boosting data use and therefore revenues. The cash will be used to extend to Windows Mobile, iPhone and Android. They are looking at a million registered mobile users so far.

Nimbuzz offers free mobile VoIP, conference calling, IM and group chat and photo and file sending across multiple IM communities, including Skype, MSN, Google Talk, Yahoo!, AIM, Jabber and ICQ, plus 23 social networks, including apps/widgets for Facebook and Myspace.