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.