The same rules apply to Snapchat, but keep in mind the cost per user for Snapchat are minimal. A few photos kept around for a temporary period. If they have a reasonable amount of smarts, the cost for doing that, especially at the scales they can now purchase, will be negligible. The revenues they can get from each user in terms of advertising is highly likely to sustain them, and their large user base means a large revenue base.

Thus, in their specific case, it's hard to make the case that it's a bubble. And if they are bleeding users away from Facebook, then FB will pay a premium to stop that bleeding. Thus the FB offer is not unreasonable, no matter how ridiculous it looks on the surface.

Now, if their user growth slowed for some reason, then yes they will be kicking themselves for not taking said offer. It has happened before - e.g. Friendster, etc. But those are usually self-inflicted wounds. Assuming their investors keep them from such moves, there is a good chance they can be a large sustainable company.