Facebook is on Top then twitter, Google+ and rest for all but most users of Social Media prefer Facebook.

At today time the facebook is the major source of getting traffic to the site, because facebook have a million of users who visit the site daily and interact with people of different community. So facebook si the best social networking site for getting good index also as well as traffic.

mickal has written a perfect site list

Apart from these sites vk.com is also good social networking site, vk.com also have a millions of user.

best social sites are
Facebook
Twitter
Google Plus
Pinterest
Linkedin
Myspace

Stumble upon is best
 

Stumble upon is best

facebook, google+ are the best social networking sites

The social networking site can be differentiated by different purpose. If you need traffic then use Facebook, google plus, twitter, linked in etc..... They have a huge traffic and also good for entertainment purpose.

Google Plus, Linkedin, StumbleUpon, are the best Social Networking site.

Facebook, Twitter, Pinterest, flickr,...
Google+ is more powerful than you might think

Facebook, Twitter, Google+, Pinterest and LinkedIn are some of the best social networking sites for promoting a website and for building connections.

Best social networking site good to use are:
Twitter,
linkedin,
My space,
reddit.
Facebook.
Etc.......

Twitter and Facebook brings most traffic to my website so they are probably best FOR ME. It's all about your content and how you want to present it to the audience.

Facebook
Twitter
Google Plus
Pinterest
Linkedin
Myspace

Twitter , Facebook, Google+, Social bookmarking,Linkedin etc.