One of the challenges of a social network, especially in a declining phase, is that there is far less commercial value being generated at the same time that various sorts of costs, including attacks on the network in both technical and social/economic senses increase. High-value members abandon the network, and those who remain are either stuck (say, because of institutional circumstances elsewhere), or are actively seeking to exploit other members.
This means that Trust & Safety costs are constantly increasing at the same time that recruiting talent to serve that role becomes increasingly difficult.
What the true cost curve looks like isn't clear, but basing your statement on a constant cost based on present experience is ... probably flawed.
This is especially true at Facebook's scale.
It's probably a better example of a service that never got off the ground.
Twitter and FB could survive for decades just on the their current cash positions alone.