Openness and accessibility are among the most important internet values. Companies that create closed gardens violate these values. Many people believe that this erosion of openness for the sake of short-term profits (2-3 years) will lead to a weaker and all around worse internet in the long run (5-20 years).
The community center people are seduced by the ease of creating a FB site without paying any cash, while likely not realizing that it's harming their community by excluding people.
Of course, being too dogmatic about values is never good. But I would argue this is a case where it's good to stand up for accessibility.
"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.
(I don't use Facebook and don't like when communities build on Facebook.)