Hot much underage porn does Snapchat carry. What's the potential liability on that given (in UK) ISPs are starting to filter porn by default.

Snapchat's rose in the mass media as the app children were using to send images of their body parts. Not just that, mean/bullying messages and "pranks" too. Of course you can do those things on other apps or through other media, just Snapchat's transient messages seemed designed for it.

It's force of the platform: A reaction to parents asking to overview the teenager's activity on the mobile phone – "Can't show you the messages I've received or sent, dad, they're timebombed!" Actually I've never gotten over the idea that Snapchat was for porn, especially since their icon looks like the shadown of an exhibitionist opening his coat...

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.)