I am not sure what you are referring to by a "traditional" monopoly. Your definition, "control supply of a good or service, and where the entry of new producer

I am not sure what you are referring to by a "traditional" monopoly. Your definition, "control supply of a good or service, and where the entry of new producers is prevented or highly restricted," sounds like the definition of any monopoly which seemingly covers Facebook and Google. Monopolies are extreme examples of lack of competition in a market. One way non-competitive market equilibriums occur are when barriers to entry for potential competitors rises. For Facebook and Google, the market externality that contributes to their monopoly-like market power is known as the Network effect (https://en.wikipedia.org/wiki/Network_effect https://en.wikipedia.org/wiki/Network_effect). This effect was first observed with the rise of the Bell System which ultimately required anti-monopoly intervention (https://en.wikipedia.org/wiki/Breakup_of_the_Bell_System https://en.wikipedia.org/wiki/Breakup_of_the_Bell_System). For Facebook, the network externality is a bit more straightforward since it is a first order effect in that new users looking to adopt or join a social network choose to join the one which has the most other users they are interested in (typically friends, but could also be public figures). This tendency becomes a self-reinforcing trend as more and more users join the single largest network. This dominance in users attracts advertisers which are the primary source of revenue for Companies whose primary product is a social network. For Google, it's also an information play. The more users they can have an interaction with the more data they can scrape to feed their advertising engines. Once again, advertisers are incentivised to pay the best/biggest data troves which gives Google the ability to invest even more in providing and improving services to attract users. Which again, becomes a self-reinforcing cycle. I wouldn't say that user loyalty/user's reticence for change is not a contributing factor, I think it is; I just don't think it would be correct to say Google and Facebook aren't the beneficiaries of monopoly-like market equilibriums. It's a stretch to call them true monopolies (actual singular market suppliers), but their market control and power are rapidly approaching, if not already equal to or greater than, that of historical examples of monopolies and targets of anti-trust legislation.