They're exactly the same. They sell advertising space on their own websites and get attention by giving away free content and services. Publishers also buy each others tech and advertise on each others sites.
The business models are identical, just with vastly different scales.
They are platforms and content marketplaces (where the currency is attention). This is entirely different than run of the mill publishers that can't drive attention at scale.
Google and Facebook can leverage their power to put entire industries out of business.
They are ad platform providers which gives them a totally different position of power than 99% of companies out there trying to live on ad revenue.