It is just as free to use as any other website with ads is.

That's actually not true. Most websites with ads don't have a huge network effect[1] that causes and strengthens a monopoly with the potential for abuse that that brings with it. Also, most websites with ads are not in a position to create a comprehensive database of the social structure of the world.

[1] https://en.wikipedia.org/wiki/Network_effect

Cool. It is still free to use though.

No, not if you are using any economically sensible definition of cost (and thus of "free").

"I have to directly pay x units of currency" is not a useful measure of cost. There are lots of ways in which either you can be paying costs indirectly, or in which other people are forced to pay costs caused by you, neither of which really makes much sense to call "free".

Network effects are what economists call an externality: A cost (or a benefit in the case of positive externalities) that is paid for by people who didn't participate in the decision that led to that cost. Also, monopolistic structures and concentrations of power often have long-term consequences that lead to what economists call opportunity costs: A monopoly that hinders innovation, for example, might end up preventing a beneficial technological development, or even just lower prices due to competition.

That people often confuse "immediate payment" with "costs" is exactly the misconception that companies like facebook like to use to hide the costs.

Communities are choosing to use it largely because it's free, so I think it's fair to use the term in this context.

No, communities are choosing it because they think it's free. That's exactly why perpetuating the myth is such a bad idea. Just because people think that it is, doesn't make it so.

It is free to them. It literally costs them no money. That's why they're choosing it. They're not fussed about the cost to their users.

(I don't use Facebook and don't like when communities build on Facebook.)

No.