Facebook is not a data monopoly, unless you take "data monopoly" to mean "has a set of a data that no one else does", in which case everyone is a data monopoly.

Nobody else has a comparable dataset on a comparable scale. Microsoft wasn't the only company supplying operating systems in the 1990s either.

Comparable dataset of what, exactly? Of Facebook's internal user data?

Think about how much data Amazon has about all the suppliers and products they've sold over the last twenty years. Should they be forced to hand it over to anyone who wants it just because of the fact that its size and exclusivity makes it very valuable?

What about the governments databases on everything under the sun? Should all of that be public?

Maybe the answer to the above questions is yes, I don't know, but I don't think it's axiomatic.

Nobody is suggesting everyone has a right to Facebook or any other company's data (just like nobody seriously suggested MS shouldn't be able to impose any restrictions on OEM Windows installations). They're suggesting that if a company sets itself up as a provider of data and other platform services to encourage everyone else in the market to become dependent upon it then selectively pulls the plug on the most successful companies or forces them into deeper partnerships, it's definitely harming the ecosystem. And that if Facebook has done that, it might also have fallen foul of laws which are there to stop that sort of practice.

I definitely see those arguments being made (in this thread, in fact), but that may not be the argument you're making.

I think the issue I have is with the idea that Facebook is successful, and therefore can no longer prevent their competitors from blatantly taking advantage of them. But I'm not sure where to draw the line.

If Visa and Mastercard decided for whatever reason that they wanted to kill some small banking chain and not allow payments for them or their customers to go through their network, that would be bad. But I don't think that means that they have to let an upstart competitor credit card startup have access to the huge multi-billion-dollar network they've built up, does it? Even if they might extend that network to other entities that don't compete with them?

I'd be pretty concerned if Visa introduced a Visa Mobile Payments platform and withdrew functionality from the fastest growing payment apps because the mobile payments sector broadly competes with the credit card sector. I think that's a closer analogy than either of your examples

Well, that'd be the only use case for that platform, right? Maybe a better example would be if they explicitly told companies who joined the platform that they couldn't compete with Visa's core business, and then Mastercard signed up because they have a mobile payments division. Would Visa be justified in kicking them off?

That's not what a monopoly is.