Alexa Rankings
Alexa.com is a service provided by Amazon that gives statistical estimates of web traffic and ranks websites based on those estimates. They sample traffic by having users opt in with a browser plugin, and use statistical methods to estimate the total traffic. Alexa rankings are based on a rolling average over 3 months, which means that a decline in rank often reflects hitting a peak in traffic some time ago, within those three months. In other words it is usually a lagging indicator for increases or declines in web traffic. A change in the curve typically reflects a change in trend.
Here are the Alexa ranking charts for some of the top Bitcoin and cryptocurrency exchanges.
Charts appear to show that they have peaked
All these charts have a very similar pattern of growth from 2017. If you zoom in on the top right corner, they mostly show a similar pattern as well:
Most of the exchanges, especially the largest ones like Coinbase, have not seen any decline in traffic since the start of 2017. This decline is the first for the year. But because of Alexa's three month averaging, it likely reflects a decline that began 2-3 months ago. The price of Bitcoin and other cryptocurrencies has continued to rise in those three months, with a new ATH for both Bitcoin and the crypto space being seen only in the last couple of days. The growth of user traffic has not kept up with this price increase.
We can see that again in the info pages for the top cryptocurrencies (I will exclude those which came into existence only recently, for obvious reasons).
Exceptions
There are of course some exceptions, with continued growth. It is possible that they have also peaked, but it would mean they likely peaked more recently.
Conclusion
The price of cryptocurrencies have continued to increase in the last three months, with more money in the ecosystem and increased demand for tokens. Growth in traffic to the top cryptocurrency exchanges has not kept up with the price, and appears to have peaked some time in the last 3 months. Traffic on the 'landing pages' of the top cryptocurrencies have also peaked and are seeing a similar decline.
This is just one possible indicator of an overheated market.