In principle, the advertisers could buy Google shares and influence the company's policy towards an equilibrium that is more favourable to advertisers. If a natural monopoly is a public company, the entities that depend on this company could buy shares and bend the company's actions towards their interests. Is this happening? If not, why not?
Google is a trillion dollar company. You'd need to have billions of dollars in Google stock before you opinion even mattered around the board room. The majority of the companies advertising on Google couldn't even dream of that and for those that could it's not exactly clear that this would be a wise investment.
