The only way to underniably confirm its a bubble is if it "pops".

Is snapchat worth 4 billion? Only when some buys it or it goes public. Till then its speculation and we can be here till the "cows come home" debating if we are in a bubble.

I think the best people to ask are PG, Fred at AVC etc. The ones who do this for living who are making these decisions everyday and know the markets. I don't know enough to say for sure and wasn't here for the last bubble.