The following social sites have agreements with Google to share data that can then be used as social signals which impact search engine rankings:
Google Groups – Merging into +1
Read It Later
Twitter – Twitter’s Google deal expired and has not been renewed. Google is still using Twitter to find relevant new content.
Facebook – There are rumors of a Google and Facebook agreement but that has not been verified.
President & Founder, Tru Family Dental
Marketing, Dependable Cleaners
President, Frame Destination
President & Founder, Family Travel Association