Mining on GPU's is generally not profitable, but if you don't pay for the GPU or electricity then numbers look different. Especially if you get some people that keep active tabs open for days.

Really the ROI is vs a 40MB download and a few days of R&D.

I doubt browser-based mining is even using the GPU at all.

HTML5 gives access to GPU resources. EX: https://developer.mozilla.org/en-US/docs/Web/API/window/requ...

Depends on how much each ad cost them. The main reason that ASICs are the only profitable way to mine is because of electricity costs and by running this ad they're shunting that cost onto the user of the site. In place of that is the cost to serve an ad which can be really cheap. I could see the math working out buying a lot of really long tail terms super cheap.

CPU mining is not profitable even for free. Assume they get 5 minutes of computation per ad at 100H/s. They need billions of ads a day just to get 1GH/s, which is around 48 cents a year.