Exchanging songs for ad money may be a frightening proposition for music executives who have long depended on discs sales. But more and more we are seeing start-ups diving into the free-music sector, allowing users to stream an unlimited amount of songs on their PCs – provided they don’t mind advertising.

Today, CBS’ Last.fm is joining the ranks of Imeem and SpiraFrog (and possibly Yahoo?) with its own full-track music streaming service. The free-on-demand service will be advertiser supported and has the rights to about 3.5 million tracks from all four major labels and thousands of indie labels. The labels insisted on one significant limitation, though: songs can be streamed for free up to three times each. After that, listeners will be given the option to download the track through iTunes, Amazon and 7 Digital.

With a loyal user base and the marketing support of one of the world’s biggest media companies, can Last.fm’s streaming service prove more popular and profitable than a purchase-based service such as Apple’s iTunes? Only time will tell.