Hi Jonathan
A topical and interesting question for sure.
I was at SearchLove London earlier this week, there was an excellent talk by Heather Healy of StickyEyes, entitled "Do Social Signals Actually Play a Part in Search Ranking?"
Have a view of it (halfway down the page linked-to above) as it helps to answer your question.
In summary, Search Rankings do not appear to be influenced by Social Signals, although there is some Correlation.
When content is shared (incl. socially) Links are often generated. It's these links that can influence rankings, rather than the actual social signals.
It is recommended to partake in Social Networking for brand building, influencer outreach and content sharing, rather than as a method to increase search rankings. Do it for Social's sake rather than for Search.
Hope that helps,
Regards
Simon
