Not all cryptocurrencies are alike.
Bitcoin is not a bubble, but most "tokens" (ICOs) are.
Unlike Bitcoin/ Ether - which are an asset or a platform - almost all other cryptocurrencies (often ICOs, such as Ether tokens) are based on a real business. Such tokens should be subject to valuation on the basis of real potential. Most tokens are also companies and should be valued as ordinary companies.
When we do so it is clear that in the ICO market there is a real bubble. Total cap should generally not exceeding around $20 million for most of these early stage companies, but some are being valued over a billion dollars.
Good ICOs may be worth buying when they are issued but in the secondary market there is a lot of irrationality.
Most such tokens will become worthless in the coming years - although a small fraction of them will become even bigger, as they start displacing established companies.
This is called creative destruction. Crypto technology is definitely going to disrupt finance and data industry, but in the process there will be a lot of tears shed. A LOT.
For instance, STEEM is currenly valued at $350 million by the market.
In my view, there is nothing to suggest that this is an accurate valuation. Steemit is simply not comparable with Facebook, it doesn't have 1/100th the features of Facebook, not many general people are joining it and are unlikely to do so given its drab and boring interface, etc.
Therefore STEEM is (in my view) currently over-valued by at least 10 times. I'd not buy STEEM.
I liked this article which analysed PAY: https://hackernoon.com/the-right-valuation-of-tenx-pay-4fd640698e9b