In the short term, if supply is fixed, then you can always pay to exclude competitors by outbidding them.

For instance: the reason google makes so much money is that market incumbents bid above value (they lose money) on google ads to prevent new competitors from gaining market share and then they subsidize that money losing behavior with their profits from existing customers.

Should the government regulate what prices google and facebook charge for ads because that controls whether or not startups can get their products in front of customers just as much as the pipes.