Snapchat could flash advertisements before and/or after 'snaps'. How big that ad market is would just be a guess with little data to back it up.
The big risk for Twitter, Snapchat, and some degree Facebook is the more money they attempt to make off of users, from advertising, the easier it is for someone else to start a less intrusive competitor. The cost of creating, launching, and maintaining a service like this will keep dropping.
Facebook purchasing Snapchat is just creepy. They think delete=hide.
As someone who works in the advertising space I can tell you with a great deal of confidence that SnapChat will not be able to monetize in the way you describe and make enough money (without pissing off users) to justify the price tag.
Unless they've got a very clever model that is more advanced than what you've described or that FB or Twitter have ever used it's a very long shot. Unless the owner has a personal issue with FB and didn't want their money for this reason I do not believe this was a sound business decision.
I'd love to be wrong but I don't think I will be in this case.
The big risk for Twitter, Snapchat, and some degree Facebook is the more money they attempt to make off of users, from advertising, the easier it is for someone else to start a less intrusive competitor. The cost of creating, launching, and maintaining a service like this will keep dropping.
Facebook purchasing Snapchat is just creepy. They think delete=hide.