[realityking]

I may just be very slow right now (it's late, a holiday, and I had too much to eat), but who, besides Google, are you referring to?

[rsuelzer]

Microsoft & Google. Not Apple, but certainly not for a lack of trying.

[dragonwriter]

Well, Apple tried to be a major advertising firm (and still tries) with iAd.

Though I don't know that I'd say that they are such a firm.

[freehunter]

Bing Ads.