I can only speak for myself, but I wouldn't pay for facebook.
As for whether I'd pay for Twitter: yes, if all the people who use it now continued to use it. Unfortunately, charging for it would almost certainly cause a lot of them to leave, reducing its value - which would lead to a vicious cycle. I think this is the main problem with monetising social networks that aren't targeted directly at who are prepared to pay from the outset.
Freemium is probably the way to go. The trick is finding a sufficient set of value-adding features, ideally introduced immediately as a premium option. Nobody likes to pay for something that used to be free. Of course, if it was easy, they'd already be doing it.
Clearly, there must be a free account available because it wouldn't be worth a penny if it didn't have as many users as it did. So I re-framed the question as "What parts of Facebook are worth a premium?" as well as "What parts of Facebook would I pay for?"
Unfortunately, the list is pretty short. I used to use Facebook a lot during college, but now I use it for the occasional info or photo lookup. When I'm bored waiting/traveling/etc, I will usually whip out my iPhone and comment on a bunch of feed items and that's always worth a few laughs. But would I pay for that? I just really can't see myself needing it badly enough.
What "advanced" or "premium" features do all of you use? What would you pay for those features?
("Sorry guys, we can't afford it anymore! We need people to chip in $1 each a month or we have to shut down. These friends of yours have already agreed to stick around: XXX, YYY, ZZZZ")
This is like a negative network effect where so many people are sucked into this black hole that it's hurting everyone.
I'm exaggerating a bit but I do see a negative effect. It may be outweighed by the positives but it's still there.
People log in to facebook precisely because people do 10 status updates a day, and post an ungodly number of notes, and join a ridiculous number of groups. If you restrict these, even if some people are willing to pay, the network is no longer compelling, and usage will drop, and you send yourself into a death spiral.
Clearly facebook's monetization does not lie in charging for an account, that is IMHO one of the dumbest ideas I've ever heard.
Oh, and the author is completely off his rocker if he thinks you can get 5% of people to subscribe to a pay-facebook. Think 1%, max.
Of course it turns out that didn't actually work for most of the companies who tried. There has to be some kind of specific value proposition or you won't get even 1% of any market.
(Okay, a bit more than 5%. Maybe 10-20%.)
They should become a media company though.