Mobile VoIP and IM service fring, one of the more popular mobile communication services around, is experimenting with porting a bunch of third-party apps to the fring client and will be announcing a number of those in the course of this year. Today, fring is launching a custom mobile-optimized Last.fm music radio add-on that brings a lot of the music recommendation service to the fring client but also integrates with the latter’s social and communication features.

Fring users (also referred to as fringsters) are now able to fire up streamed music radio channels, including their own Last.fm library, tag favourites, ban disliked tracks, view album artwork and basically take their Last.fm music account mobile. In addition to getting those features into the client, fringsters can also opt to view in real-time what their contacts are currently listening to, with the help of a friends list embedded into the Last.fm add-on.

Like Last.fm’s own Android application, which it introduced just last week, the add-on can be run in the background enabling users to listen to streamed music channels while they’re using other fring features.

Third parties can built add-ons on top of fring using its API, for which you can find more information here.

The Israeli company, which recently laid off 20% of its workforce but continues to surge with on average 400,000 new downloads per month, has made it public that they’ve raised 3 rounds of financing so far, although they’re only disclosing two rounds of funding amounting up to $13 million for now. Although they’ve recently started showing ads to mobile users, the jury is still out if there’s enough sustainability in the business model for the service to keep them alive.