Jack from Google said in a Google Webmaster Help thread that adding a Facebook Like Button to your web pages will have no influence on your Google rankings.
The response needs to be put in context. One webmaster claimed his rankings dropped soon after adding the Facebook Like Button to his web pages. In response to that, Jack said - no, it wouldn't drop.
The thing is, we know that Google uses site speed as a ranking factor. We also know that Facebook Like Buttons, if implemented certain ways, can slow down the load of your page. Does Google specifically make sure to ignore the Facebook Like Buttons in the speed tests? I don't know. But if not, and it slows down your page load time, it may negatively impact your rankings. That being said, it is not specific to Facebook, anything that slows down your page can be detrimental to your rankings.
Going a bit beyond page speed, we also know Google uses social signals for rankings. Right now they use Twitter, Google Reader, and other social sites but currently do not use Facebook. So Facebook has no real direct impact on rankings as of yet.
Forum discussion at Google Webmaster Help .
