I think there are two quick answers to the question:
1.
SEO plays its part in Web 2.0 sites as much as anywhere else. Sure, if you know the URL you can reach anything on YouTube, but this is no different from finding a site on the web. But the key is to be very well placed on any source and this depends on content, content, keywords, keywords, etc., ad nauseum.
2. Web 2.0's audience is less business oriented and, on the whole, a younger one. As has been observed, you're dealing with a Social network. Thus, whether you think
SEO is necessary or not, it's abolutely vital if your market is, as it were, non-social searchers.
Duncan