IMHO Quality signals from social have been gaining in importance for search engine algorithms. It makes sense. Search engines will seek out any quality signal to help them determine the most relevant and useful serps.
Historically search engines have looked at backlinks, keyword density, heading tags and many other data points in the algo. Webmasters are very good at figuring out what signals will lead to a boost in the serps. This has lead to these signals being abused and manipulated (ie paid links). Social has not been abused as much. IMHO it is a cleaner signal and possibly more accurate in figuring relevancy.