How Many User-Generated-Content Creators?

If you have used Orkut, MySpace, Facebook or blog or edit Wikipedia, then you are contributing to the User Generated Content on the internet. So, how many people like you are out there? For USA,

2007 : 77 million
2008 : 83 million
2009 : 89 million
2010 : 96 million
2011 : 102 million
2012 : 108 million

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