Depends what you mean by stealing. I definitely remember some large services (like say, YouTube) originally becoming popular based on stolen content posted by users and there's a lot of controversy about Facebook seemingly profiting off users reposting other people's videos on their site, but generally it's not the site creators who are stealing the content.

But yeah, audio and video hosting sites tend to get popular or start out on the greyer side of the law before trying to go legit.