Criminals use more dollars than bitcoins. The currency can't help being used by criminals.
How can you know that? if ever there was a survey that happened. Because I do not think anyone knows for sure criminals, more use dollars or bitcoin. But I think because the dollar is better known, the majority of criminal use this currency. IMO
Its not about surveys, but the amount of money estimated in all worldwide criminal activites exceed Bitcoin capacity by a lot - the Bitcoin blockchain dont have such wealth transfers recorded even if Bitcoin was used just for criminal activites, which it is not of course. Probably majority of people (including criminals) going to use Bitcoin in future, simply because of Bitcoin better properties, but now its under 1%.
I wann bring here too forum,but he said he not neccesarry about forum,because he said he's know about currency :/