How many use Selenium specifically? No.

It was just a random example that was intended to not point in an easily weaponized direction.

I figured it was both common and known in tech circles that Flash/Javascript executing botnets for ads existed and have for year(s):

http://www.spider.io/blog/2013/03/chameleon-botnet/

They were acquired by Google so it isn't just random blog spam:

http://techcrunch.com/2014/02/21/google-acquires-spider-io-t...

Installing Selenenium on infected machines is not very stealthy. The new malware usually tries to stay in the kernel to avoid detection. Try stuffing Selenium in the kernel :-)

My personal experience with comment/forum spam bot networks is that at least 90% do not understand javascript. The ones that do tend to have extremely realistic traffic patterns and the ability to solve captchas. I suspect those are actually real people with incredibly crappy jobs, not super-intelligent Selenium-powered bots.

Comment/forum spam is different game than botnet ad fraud. The latter executes JavaScript by design since it's often necessary to load the ads.

see the charts from Incapsula in this article which show a third of the bots are not executing javascript https://www.linkedin.com/today/post/article/20140623200557-8...