The test will be if they can charge for membership. People find phone service worth paying for. People find internet service worth paying for. Some people even pay for TV.
Would 50% of current users pay for Fbook?
However:
• As a customer, my demands are much greater than as a user. I feel more entitled to make demands and get real attention than for a free service. Customer service becomes more important when money is involved.
• I would expect absolute control over my own data. No advertising. No selling aggregate data to advertisers. If I quit the side, my data needs to be gone completely, without a trace.
• If my friends started quitting the service in droves, due to fees, I might no longer consider it worth the money and would probably leave or wind down my involvement as well.
A paid Facebook would likely look different than today's Facebook, both in features and in financial organization, but it may be a more predictable and sensible investment.
My demand would then be data security, no advertising. Other users could stay on free service, but the apps they use shouldn't have access to my data anymore.
EDIT added mention to invisible votes
Facebook has had reasonable success with apps. But the whole app use/discovery/interaction/development experience on facebook has a lot of room for improvement (euphemism to mean it's terrible). And they do seem aware and willing to improve on this arena.