1085
...Granted you can turn this off, but by default every time you start Steam an ad for a game flies up in your face.
I would also call every single store page on Steam a "sold ad." Again, granted that it doesn't seem you can pay to promote your game above anyone else's and the search seems to be fairly straightforward and functional.
While I do feel there is definitely advertising that happens on Steam, I'm okay with the level of it. I can find products I want, and products I do not want are not mercilessly crammed down my throat.