If Google and Facebook use this policy to establish an oligopoly (or cement their existing one), and that allows Google and Facebook to raise advertising prices

If Google and Facebook use this policy to establish an oligopoly (or cement their existing one), and that allows Google and Facebook to raise advertising prices above equilibrium pricing, then that squeezes companies that wish to advertise with costs that are shifted onto consumers. Additionally, having a Google/Facebook oligopoly hurts incumbents when Google uses its overwhelming advertising market power to give itself exclusive advertising deals in order to promote some product. Advertising monopolies can be used to prop up and establish other monopolies. The most obvious example of Google doing this is advertising Chrome on the front page of google.com.