So, clearly it pays to tweet and build a credible Twitter presence. This means gaining a significant following in number, but also getting authoritative users to follow you (and hopefully retweet your links). This would appear to be good for plain old fashioned organic rankings as well as other supplemental search results. More importantly, it pays to create good content that will attract authoritative Twitterers to share it with others.
These things of course pay anyway, but it's nice to know that they actually do have an effect on search rankings as well.
It's not surprising that Twitter is currently playing a more important role in to search engine ranking than Facebook, simply because Twitter is public by default. Facebook is much more walled, meaning that most of the good Facebook data is private. Though it will be interesting to see what happens when Bing does integrate its newly acquired Facebook into its regular search offerings.
This wall of Facebook's is also a factor into why Facebook could potentially unleash its own legitimate search engine. It's got a search feature now of course, but there is a great deal of potential for them to do a whole lot more and really get under Google's skin. More on that here.
Sullivan provides more background and context around the search engines' social signals, as well as the full with both Bing and Google, and details about the Twitter Firehose's lack of nofollow.