Facebook can't raise taxes, monopolise the use of force, grant children of its netizens Facebook netizenship, and so forth.
I'm not saying that there never could exist a state that exists purely in the online realm – in fact, technologies like Ethereum makes me think we're not far off from net-states existing.
Their 'taxing' of netizens takes several forms, mainly displaying ads and collecting and selling information about them. Both of these are profitable to Facebook (or they wouldn't do them), and injurious to users (in lost privacy and attention). Facebook can choose how much of it they do; they balance taxation (profits) with user retention (emigration).
Facebook have a monopoly on many kinds of force on their platform. They and only they can remove or block content and ban accounts. And unlike other states, there are no courts, no constitution, and no way for rebels to gain these powers for themselves by force.
The question of granting netizenship is less important, because Facebook encourages immigration: they'd love all the world to be their netizens! But they can exile people they don't like, or block them from joining.
Facebook also have partial control of the presses in their country and of the commerce at its borders (in- and out-going links).
Facebook's business practices aren't a "monopoly" in any usual sense of the word and so once again it is just making it difficult to discuss the associated concerns/problems by expanding the meaning of "monopoly".
Facebook can't levy taxes on its citizens like a state, but it can raise revenues. They can't realistically monopolize the use of force, but that is due our current social climate. In the past, the Dutch/British east india companies, Belgian Congo and other early trading corps had their own standing armies. More recently, agricultural corps in Latin America employed their own paramilitary groups. If there was ever a need, they could create a PMC.