There is a subscription only thread at WebmasterWorld that has one SEO claiming he was able to increase his site's ranking only through Twitter.
What he did was send hundreds of Tweets over the timespan of two months. He tailored the tweets using @ replies to hundreds of people, with a short URL pointing to a specific blog posts.
Then his site's ranking jumped from the fourth page of Google to page one, rank number 5. Now he is ranking in the number two position.
He said there was nothing else done.
Of course there are many questions in the thread and others reporting similar cases, even with Facebook. Of course, all speculation but it is a good thread.
Again, you need to be a member to see the WebmasterWorld thread.
Forum discussion at WebmasterWorld.