Since Google introduced "Search Plus Your World," most Google Search users have seen results that add more emphasis to sites shared on Google+.
One search engine marketing firm, TastyPlacement - http://bit.ly/KBn2qJ, took it upon themselves to conduct an unscientific-ish study to determine just how much of an effect Google+ promotion has on search ranking.
It turns out having Google+ followers boosts the ranking the most, while a "+1" still does way more for your search ranking than Facebook or Twitter.
Twitter actually ranks the lowest — which, given the bickering between Google and Twitter, isn't all that surprising either.
Read more: http://read.bi/KBnIwo ;
Via Steven Healey, John van den Brink, janlgordon, Andy Bull