Social media's Achilles Heel?

I read a mentioning on GigaOm about Hulu's decision to set up a YouTube channel. Although YouTube and Hulu are not head-to-head competitors but rather only relate to the same audience, it still got me wondering about competitor relations in the user-generated web realm.

So, are consumer generated sites vulnerable to competitor infiltrations?

Traditional businesses have the ability to keep product information out from the competitor's hands. Like the time I went to buy furniture. The store manager almost kicked me out twice, because I took a snapshot picture of a sofa I liked to send to my girlfriend through my mobile phone. I was not a competitor; just merely a guy wanting his woman's approval for a high priced acquisition :) Nevertheless, the store would not permit it, and so took active measures about it...

As YouTube, and other UGC sites, accept commercial involvement, they may be at risk just by allowing so. Well, I'm not so worried about YouTube, but actually the other way around: a promising site that is gaining momentum can suffer by big sites targeting its audience.

Can social network sites protect themselves from this, and more importantly, should they?