I use google except for the rare few times I have been on my phone and google wouldn't pull up. It's strange because it's a droid phone and you would think of all websites that would work, it would be that one.
Yahoo is the #2 website in the world, so even though a lot of people use Google there are still plenty out there using Yahoo. I know Bing is connected to facebook (don't know if your advertising crosses over, probably not), some non-droid/non-iphone phones (specifically, Samsung) have Bing built in, so it has to have some users. I don't know of a lot of people using alta vista or any of the other older search engines or the smaller engines.
I would assume whoever you are advertising through should be able to provide analytics on where your traffic is coming from (if they won't, I'm sure you can install google analytics) and let you know whether you are wasting your money advertising elsewhere.