Twitter is teaming up with Apple (and an Apple search engine may just happen soon)
Facebook is already tied to Bing...
Pinterest generates NEGATIVE results (try it, you'll see)
There may be other signals, but I doubt it.
It was obvious why my tests failed to produce results, however. Google is not going to give their competitors (FB/Twitter/Apple) the ability to influence rankings in their engine. If they did that, it'd only make them more powerful.
Google and Twitter are not partnered at all by the way
http://searchengineland.com/as-deal-with-twitter-expires-google-realtime-search-goes-offline-84175
If social media offers any more than a tiny, tiny fraction of Google's algorithm, it will create a spam war of epic proportions. Generating tons of retweets and such is so damn easy it's not even funny.