Read about it in Danny Sullivan's post on Searchengineland here :: What Social Signals Do Google & Bing Really Count?

Here's a video where Matt Cutts Confirms it(In May, Google had mentioned that these links weren't used for any ranking-related purposes) :


Few Things worth Mentioning ::

1) You can now Use twitter & Facebook(to some extent) as a form of Linkbuilding.

2) Authority of a link emerging from twitter is measured by Google & Bing on the basis of your Twitter Pagerank, Followers & the number of Retweets of a particular tweet where your link is included and some other factors.

3) Google does measure Authority on Facebook too unlike Bing. They haven't mentioned much on what factors are considered.