I would be very interested to know this too, but of course it's all technically against the ToS[1] and so I doubt Snapchat will ever even acknowledge it, much less give numbers on it. Don't want to offend those delicate advertisers!

[1] http://www.dailydot.com/debug/snapchat-porn-banned/

(The article is over a year old, but not much has changed since then.)