All things are bubbles. House prices, stocks, domain names especially, even bread nowadays. Hardly a benchmark of distinction there.
The price has gone up because the blackmarket has discovered the usefulness of bitcoin. Drugs [mail order], money laundering etc, quite frankly why anyone still questions the demand for this sort of currency is beyond me.
The demand hasn't even hit anywhere near like a peak. Key issue is can the authorities put a stop to it, which at the moment doesn't appear so.
And second issue; is the currency stable and secure? [Who knows]
But 500million isn't enough to get the feds off their leather sofas, it's when it becomes a multibillion situation that things might get interesting.