Startup data trends (which gets its data via AngelList's API) shows 18799 startups in total when you search on location "Earth" http://startupdatatrends.com/?tags=1643
It's reasonable to think that most of these startups created AngelList accounts in 2011 (given hornbaker's Alexa graph).
I would read that as 500 startups that got funded. I imagine there are WAY more startups that signed up.