Here is from Reddit: http://blog.reddit.com/2013/01/aaronsw-1986-2013.html

Depends on your definition of founder I guess, he was a founder in the sense of "founding father", but he wasn't there on day one. He did join early on and had equal equity in the company with the other founders though. Since I don't know the best way to describe that, I'll just say "pseudo-cofounder".