I think there are some great answers here, hopefully this isn't too repetitive...
First, the metric you gave (2m daily unique visitors) is a great number, but it's not the right one when you're talking about proof-of-work mining. In that world, the relevant metric is more like the number of CPU-seconds you have available to you (a "cycles per second" number).
To be more specific, your number doesn't really cover what's going on, because one unique user could leave their browser open all day (and therefore hashing for you all day), while another could close it in one second. Those two users would do vastly different amounts of hashing work, so you need an idea of the number of concurrent tabs open.
Using the awesome numbers from @Nayuki's answer, we can get a bit further:
(Note, I'm using the "Hz" unit here with one "hash" being one cycle.)
- each open tab to your site could generate about 30 MHz (per the calculations above)
- the BTC network is doing 800 PHz (800,000,000,000 Mhz)
- A big mining pool (ghash.io) is doing about 7.5 PHz (7,500,000,000 MHz)
- 7.5 THz / 30 MHz per concurrent open tab = 250,000,000 concurrent open tabs
This is... actually not all that ridiculous of a number. It basically means that to hash as much as a large mining pool, you need all of the US on your site all day every day. This is really tough for a single site, but a global ad-network serving ads that just do hashing could potentially do this, as I suspect there are at least 250m tabs open at any given time if you're looking at the whole world.
That said, you'd almost certainly earn more money per user relying on advertising, so I don't expect to see ad networks go this route anytime soon.