Last.fm's free, ad-supported music streams have resulted in a 119 percent increase in music purchases through Amazon, the company said today. The service, which launched just over two months ago, has also brought on a number of new users, although old users are contributing to the trend as well. Last.fm believes that, while the service is still young, the proof is in the pudding: allowing users to have full-track previews drives music sales, both digital and physical.

When Last.fm launched its on-demand music streaming service in January, it made millions of tracks from all four major music labels (plus a smattering of indies) available for free. The labels get paid every time a track is played, although there's a limitation of playing a single track only three times before being shown an ad and links to affiliate partners where you can buy the music.

Those links are apparently paying off. Although Last.fm has affiliate partnerships with iTunes and 7Digital as well, the company did not release sales data for anyone but Amazon. And while music purchases through Amazon more than doubled after an influx of new users joined the site, Last.fm says longtime members were buying more music too—there was a 66 percent increase in album and track purchases among existing users.

"In just over two months it's become clear that people will buy CDs and downloads if they get access to the kind of service we offer," said Last.fm co-founder Martin Stiksel in a statement. Stiksel went on to tout Last.fm as the leader in offering free, streaming music from major labels, although other players (like MySpace and imeem) are getting in on the streaming music fun too. Surely the music labels are happy, too, that the increase is affecting CD sales in addition to cherry-picked digital tracks, as CD sales have been a bit anemic as of late.

If music sales as a result of streaming offerings show growth over a longer period of time (say, a year), then other services may also begin to push for full-track previews in hopes of increasing sales. Imagine if Amazon MP3 or iTunes allowed full previews on their respective services before buying—digital music could take off even faster than it already has. It shows how the very ideas that the music industry resisted for years have the potential to pay off financially.