A huge number of people would drop off of FB, and would never try quora, if they charged. Fewer people on each makes each less useful to each potential new user, and to every existing user, causing more people to drop off.
At some point the pressure is on to join, however, because everybody else is on it. Like having a cell phone: people expect to be able to reach you, so you are pressured into having one (even some old buggers who swore they never would finally succumbed).
So while I agree there would be a loss, I'm not so sure about "huge." Would people be pissed? Yes. Would there be backlash? Absolutely. Would it open the door wider for a competitor? Probably. But they would be making honest money, more than they are now, and could innovate.
I may be wrong about its chances for success, but to me it's not obviously a failing move.
I doubt that this is related to making money.