There are hundreds of ways to get free music that haven't had the same explosive growth rate as turntable. I think the article is right in that they've done very well with subtle but surprisingly deep social aspects.

I do think they have a huge legal problem looming, especially with regards to letting you play tracks others have uploaded. Google and Amazon are already fighting the 'public performance of copyrighted works' issue, and turntable is far more blatant.