I'm not so sure the term "bubble" is a good one.

Since Bitcoin is so many things - currency, commodity, payment system, protocol, internet money -  you can't make direct comparisons with say; the real estate market or the gold and silver markets.

IMO Bitcoin has several of the qualities of an Internet startup company with new/revolutionary technology. And some startups just keep gaining value until they are multi-billion dollar companies...

The fact that valuation is rising fast does not necessarily imply that there is a bubble.