There seem to be some logical considerations that make the growth of advertising self destructive for advertisers. The big one is that the more ads one sees, the less likely they become to interact with any given ad. There will also likely be an increased ability of users to ignore the ads they do see. As advertisers see returns from advertising decrease, the costs for advertising will gradually trend downwards. This will enable even more advertisers to start advertising.

It's almost like a sort of reverse network effect. The bigger you get, the less desirable your product becomes. And the advertising industry's getting quite large.