There is a Facebook group called "I WILL NOT PAY £3.99 A MONTH TO USE FACE BOOK FROM JULY 9TH 2010!"
I note that the name is all in upper case and carries an exclamation mark. Thus it is easier to believe that it's rooted in semi-infantile hysteria rather than sound inside information. None the less there is interesting and simple maths that can be done.
The "I WILL NOT PAY ..." group has almost half a million members - this out of 350 million Facebook users. So, a company that bought Facebook and started charging for it might expect 1 in 700 users to become upset and stop using FB. It's an addictive site though, a good many of the half million may be making empty threats, particularly people who have put loads of time into fecking Farmville.
None the less, let's assume that slightly more than 1 in 700 would leave. For the sake of convenience let's say - 359 users out of every 360 would quit - leaving a million paying users. Let's only charge them $1 a month - then we're raking in $12 million a year before even charging for advertising.
Of course I haven't the slightest idea of if or when FB might turn commercial. Contrary to popular wisdom though, there may be danger in numbers.