2) The only realistic way to short BTC is to do so via a very small list of exchanges. Even if you trust them (which, given the history of cryptocurrency exchanges is hardly a given) the borrow cost is huge: there's no way you could reasonably make this kind of macro BTC bet.
The combination of these two realities makes the "If you don't believe the price, you ought to be shorting it" assertion completely baseless.
(In my personal opinion: It's like asking when the next collapse in a sandpile will happen & how big it will be. Not possible.)
Because this is a self feeding cycle, the price can double & redouble from almost any point if the "story" is good enough. So you can be dead right about a given asset being in a bubble, but if you short it the odds are that you're going to get painfully stopped out long before the bubble collapse finally happens. As Keynes said, prices can remain irrational longer than you can remain solvent: The history of investing is littered with individuals who were right, but lost their capital because they were simply too early.
“The price of BitCoin has a 91% correlation with Google searches for BitCoin” http://uk.businessinsider.com/bitcoin-price-correlation-goog...
Obviously BTC can be in a bubble & whilst still having real value - the two things are not necessarily connected. (Although having a good future value story is helpful for a bubble to form in the first place I would argue.)