There are various ways ads could be regulated and historical precedents.
Regardless of whether this ever happens, currently, online advertising is highly centralised and under increasing scrutiny. This centralisation is not the result of innovation by a select few companies but plain old M&A by the so-called "tech" giants, while the the government was apparently sleeping. If more competition were to take place in online ad services, this could potentially have a serious effect on the lifeblood of so-called "tech" giants. Time will tell.