The highest correlation with Google organic SERP Rank

Presenting the correlations using Spearman’s rank correlation coefficient, this newly released research data was used to determine that Google +1's (0.4) have easily the highest correlation with Google organic SERP Rank. This is closely followed by number of backlinks, Facebook shares, and Facebook total (summary of shares, likes and comments), each at 0.34. The same study published last year, looked at 21 factors, found social signals occupying 5 of the top 6 spots.

http://www.automotivedigitalmarketing.com/profiles/blogs/google-makes-social-signals-top-factor-for-organic-serp-rank