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In a recent video Matt Cutts mentioned that Google now uses social signals from Facebook and Twitter as one signal from 500 other signals used for Google Algorithm.
But, he also mentioned that, Google has only access to publicly accessible pages in Facebook. Most of the Facebook Pages are actually those pages. And practically, these pages are actually praising their links and their activities. They are talking a little about activities of others. So, Facebook as a signal is not as strong as Twitter.
While Twitter is wide open and also the leader in real-time information dissemination. Google also mentioned that they crave for real-time information. I have heard Google working for Twitter spams and scoring good tweeters and identifying bot twitters. This was actually the start for Google to use this vast social signal. But, Google has always been a lot less interested about Facebook.
So, the scores from Twitter counts the most, while Facebook does have influence. But, I am sure it is not as large as Twitter.