Yes, I would, at least for Twitter (I don't use Foursquare). I would also pay for Facebook, Gmail, Google, Google Analytics, Hacker News, and a whole slew of other sites that are free.

I'm sympathetic to the (probably true) view that they never would have grown to their size (and usefulness) if they were paid services, but that doesn't mean they can't just flip the switch today.

As someone who runs a freemium site, I know how important real (non-ad) revenue is. I also don't mind paying for good quality service or content. Finally, paid services have fewer scammers and bots.