Blogs were the social web. Friendster was the social web. MySpace was the social web. Twitter was the social web.
With the possible exception of blogs, these are all walled gardens. I'm not disputing the statement, but we now know these are bad places to grow the Social Web.
(I say possible exception because, while with blogs you can self-host and if you don't want to do that, there are multiple options to choose from, you can still get caught in the trap of trusting one provider and losing everything/getting locked out. Thinking about Posterous here.)
So if not the start date, 2008 is still an important milestone - it's when we started cutting the cord from these walled gardens to grow an independent web.