. . . why not though? I don't see how permitting monopoly rents in perpetuity is an economically or socially desirable outcome.
Monopoly is defined loosely and based on the idea of market power, not being a sole provider. The Wikipedia article you cite even mentions Microsoft and Intel as examples, and they had less market power over their fields than Facebook does.
What's more, this is a conversation around individual users. Facebook's economic rent is paid by their customers, which are advertisers and not the subject of this conversation.
Except it is, because it's able to leverage its market power to dictate terms to the market. If you prefer the term "oligopoly" rent you can do that, but there isn't much point. Their network effect and capacity for lock-in allows them to collect economic rents, which is the point here.
>Facebook's economic rent is paid by their customers, which are advertisers
No. Loss of privacy, surrendering your data and information, and platform constraints are all forms of rent paid to Facebook by consumers.