That's a tautology, isn't it? People found the German Papiermark useful for criminal and non-criminal activity, which meant that it was worth something. Until it wasn't.
An old billionaire who has been around long enough to see fly-by-night scams come and go, Charlie Munger, called Bitcoins "rat poison". The only problem with that is the commodity of rat poison has some actual value.
In the past 20 years we have seen various things with inflated values. Homes with sub-prime mortgages in 2007, stocks like Webvan and Pets.com in 1999. Bitcoin, which is trading for $775 now, and was worth $1020 last week, is one with those things, as well as with Dutch tulip bulbs and the like.
Comparing a Bitcoin to any other commodity, I have to ask, what use value does it have? It has none. Gold is useful because it can fill teeth, conduct electricity etc., I can also easily use it to trade as a currency as it is durable, portable, divisible and uniform. Bitcoin is traded, but has no underlying usefulness like other commodities.
Sometimes people fish around for a comparison to something with worth and are hard up for one until in desperation they point to the only thing they can - the dollar. True, the piece of paper dollar never had intrinsic value. Until August 1971, it was convertible to gold. The US did and does store thousands of tons of gold at Fort Knox and in other places. In 1971, it temporarily (with exceptions for some people) stopped that convertibility. So the dollar now is doubly abstracted, it was an abstraction of the gold, then with the window closed it became an abstraction of that abstraction. The US pays a lot to guard those tons of gold that still implicitly (instead of explicitly) backs its currency. Bitcoin does not have tons of gold implicitly backing it. It is inherently worthless.