[j-wang]

That's an interesting question since advertising certainly follows search dominance, but it doesn't necessarily follow the other way around. Google figured out how to monetize its dominance with advertising before it had the behemoth ad platforms they have today. It's pretty much the same with the popular social networks.

The answer (more logically) should kind of be neither. Advertising obviously has a lot of channels, and even though Google has both Adwords and its display advertising network, it doesn't follow those really need to be the same provider... at least outside of more data to do more targeted ads. But, again, advertising dollars will follow platforms that price for ROI.

Better targeting mainly adds to the amount that Google and Facebook can charge for their ads and still have companies pay for them. It doesn't really add to their dominance directly (I say directly since, obviously, more money can buy more R&D/employees/regulatory capture/acquiring competitors/just-paying-for-dominance like with Google paying Apple. But that's all indirect).