HOW many users?
Social networks are more popular than ever before and user/usage statistics for some of them are mind boggling. In particular Facebook with 800 million users - how long before a 1 billion? - Google+ with 40 million after launching as recently as 28 June and lesser known Scribd with a huge number of regular readers.
Below are recent stats together with sources. Many thanks to @libertyhumanity for providing the research.
Facebook - 800 million users 
YouTube - 800 million unique user visit each month 
Twitter - 100 million active users 
LinkedIn - 135 million users 
Scribd - 50 million monthly readers 
Google+ - 40 million users 
Foursquare - 10 million users