How much, if at all, does Facebook impact Google rankings?

The short answer is, "None of us know". The addendum to that answer is, "Whatever's going on, it probably changes every day. Or several times a day."

While there is high correlation between Facebook shares and top search results in the SEOMoz Ranking Factors report, correlation is not causation. I think Rand and the SEOMoz crew would and have been pretty clear in that. Good content is often cited in many ways and content with many FB shares is likely to be heavily linked to as, well. Social is just another way to cite content.

So, while we don't know the weight, I think all of us who have been doing this for awhile agree that social shares, particularly Facebook and Twitter, mean "something". I tend to believe it has very little weight in the rankings but a heavier weight in the allocation of crawl budget.

While everyone wants to hop on the Social Bandwagon, let's not forget that social activity doesn't necessarily refine the quality of a process. In fact, the more people you involve in a "decision", you're just as likely to skew the quality as were to have improved it. There are lot of people I follow or who follow me how have vastly different cultural, ideological, and other points of view than I do. It's really kind of silly to assume because I've "friended" somebody on a social network that this, who could be a low value acquaintance, should have any bearing on what kind of car or CRM software I purchase next.

So, I think Google is still learning how much social actually improves results. My hope is that they don't jump the gun in an effort to be more "social" and destroy the search results. They should, however, use the social signals to corroborate what is popular and make sure they expend crawler resources against that.