I see Snapchat as a faster Youtube. It's basically content that you want to see - Youtube from the masses, Snapchat from your friends.
The Youtube advertising model is clearly fantastic. From things I've read some time in the past I know they kill it on a $/user basis vs pretty much every social site on the web.
Putting a 2 second static 'brand flash' in front of every snap would probably not put people off using the service (in the same way that people still use Youtube - even though video hosting is a highly replicable thing). And from a brands point of view 2 seconds is probably all you need to get into someone's subconscious.
Bonuses for being an app:
1) Full screen take over. 2) No distractions from other tabs. 3) I'm sure they could find out age and sex quite easily if they wanted to. So they would have - age, sex and location. POWERFUL.
The sooner the better. Explain that the app needs to make money to it's users. People get used to things.