There was a speculative viral on Facebook these days speaking about Facebook becoming paid.
It was a fake viral trying, however actually it set me thinking.
If Facebook was to charge $10 per user per month, it would possibly end up doing $ 21 Billion in revenues in 2012.
I have drawn a small chart, you may have a look at:
https://neeleshpednekar.blogspot.com/2011/09/how-much-money-does-facebook-make.html
Its a spooky thought, users have no option.
Nayana Somaratna
From what I have heard about conversion of free to paid users, a more likely proportion would be ~5%. Probably even less, since the social value of FB is primarily dependent on the number of users.
More to the point, a large number of users would just move to Google Plus. Facebook cannot afford to let this happen.