Facebook

This is, without a doubt, the most preferred social networking outlet in the world. Everyone from socially active teenagers to research scholars to even Fortune 500 companies is on Facebook today. Given its user-friendly interface and a plethora of options and settings, this website has become THE virtual hotspot for Internet users. Since its launch in February 2004, it has overtaken the likes of MySpace and Orkut, and has nearly become a necessity for regular users. Owing to its popularity, it was even the sole subject of the 2010 film The Social Network. 

Currently, Facebook has around 750 million unique monthly visitors and close to 850 million users. As such, it's very likely that this is going to be the first social media site to surpass the billion-user mark!