Check out these advertising on social media sites images:
the social media universe
I took numbers from a much more comprehensive list produced by KenRadio.com and excluded those sites with less than 1 Million users, or an unknown number of users, or very demographically or geographically narrow sites. I bolded the ones that I felt had the most "buzz" but it’s totally arbitrary and limited by my personal experience. The numbers come from KenRadio.com, I don’t know where he got them.