Then everybody else would have to decide if the rest of the network still provides value to them, so a few more would disappear. At some point you'd reach an equilibrium where either a small portion of the original network is happy with what they still have for the money, or the whole network collapses.
It doesn't feel like people would quit because of the expectation that "everybody with any sense would stop using it". You could already say that now about FB, and you could definitely say it for many other products. Yet they are still used.
This question (can Facebook charge people to use its service?) is fundamentally vastly different from the one in the article, which basically is: can someone kill Facebook by buying enough people out of using it?