Check out What Social Signals Do Google & Bing Really Count? at SearchEngineLand.

Teaser:

So, while Bing and Google do have a human signal they can assess from Twitter, supposedly they have no link signal that they can also count. But as it turns out, both of them get what’s called the “firehose” of data from Twitter. This is a constant stream of what people are tweeting.
In that firehose, links do not carry nofollow attributes — so there is some link credit that counts, in some cases. Bing tells me:
We take into consideration how often a link has been tweeted or retweeted, as well as the authority of the Twitter users that shared the link.
Google tells me:
We use the data only in limited situations, not for all of general websearch.
Everything in the "Questions & Responses" section is very important reading.