When people ask me why I'm not on Facebook, I usually tell them that Facebook is like a really high-maintenance girlfriend without the upsides. Twitter is the only "social network" (if you can quite call it that) I'm prepared to use, mostly because it's so loose and informal. I hope it stays that way. I hope the "Hot Political Topics" bar isn't a sign of future feature creep that plagues other sites. (I'm not even in the US, why can't I turn the damn thing off?)
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.