I have yet to speak with anyone who thinks Snapchat's valuation is realistic and that their business model will sustain them. When I see this I just think of Twitter all over again. I would love to hear arguments supporting Snapchat though.
I wouldn't. Face it, this class of SV "startups" are the new pump-and-dump investment vehicles. They burned $3B in funding to build a chat app, and Prince Bin Talal needs to cash out with 10X return at some point.