My guess: Snapchat's VC structure gives relatively little to the founders (who are currently in control of the company) and their financial position is better in possession of the IP than after the sale of the IP. It makes sense. Facebook has been flailing about lately, throwing cash at anything they see that might preserve their marketshare. So $3b is their "safe" offer, rather than their "pain point" offer. I'd feel dumb passing up on $3b unless I suspected there was $6b on the table.
I don't think they're idiots. If we all think they're idiots, they know something we don't. A finite window? Sure. Then again, how old is Dropbox? six years? Apple offered them "nine digits" in 2009 and by 2011 they were making $250m in revenue PER YEAR. Right now, they might be making a billion a year, they might not. So. If SnapChat figures they could be making $1b a year by 2016, taking $3b from Facebook is a fool's bet. Personally, I don't see SnapChat making $1b a year by the time the sun goes out, but IPOs don't care. Twitter will make about $40m this year but Wall Street values the company at eighteen fucking billion dollars. Amazon, as mentioned, has never really made money at all but has a market valuation of $165 BILLION.