If the blocklist is manually curated then the probability of a website being blocked will depend on its popularity. I wouldn't just be interested in "X% of sites blocked," I'd look at "Sites seeing Y% of web traffic blocked" etc.
It is a combination of manual and automatic blocking. Facebook censorship is manual. Dick Cheney Wikipedia page being blocked is because they have added Dick to their automatic blacklist, so it gets censored regardless of the context.
