I think that recently the growth has been exponential thanks to all the markets. When I first started using Tor there were not even any drug forum hidden services at all, although some people on clearnet drug forums used Tor to connect to them. The first forums started using Tor several years ago, and since then it has just been a big transition to Tor, and now most of the drug forums use Tor and it is rare to find any that don't. So from the time I started using Tor to today an entire very large community of people started using Tor, essentially everybody in the online drug scene. I think the same thing is mostly true of the CP scene as well although they were on Tor before we were. But I think that they had a transitional period similar to the drug scene where they moved from VPN's to Tor to Tor hidden services. Even more recently we have started to see some carder forums on Tor, unsurprisingly they were the last to join the party. The number of people using Tor for legal things is undoubtedly increasing as well, not only is Tor being used more in repressive countries and countries that censor access to the internet, but even in countries like the USA people are more and more using Tor because of things like the NSA spying on everybody. So I think that the darknets are growing pretty rapidly at this point, hell just in the past two years Silk Road alone is probably responsible for at least 100,000 new people downloading and using Tor at least once.