Well I am using Facebook and Twitter for SEO purpose and both are helpful and effective for me. It really drives traffic to websites and helps in improving the rank of website.
Social media site like Facebook twitter, g plus, linkedin, pinterest; it helps you in building good social media reputation with more traffic, which is considered by Google while ranking your website in search results.
Social sites are only used to promote your site and increase the traffic to your site. But with the latest google update the social media links are also considered by the google and it helps in getting higher rank on google.
There are many social media sites for doing different SEO tasks, but what i know facebook, twitter, LinkedIn, and youtube is the best social media sites.