> Take Facebook for example. It's a free service that most of its users get a great deal of value from. If asked, wouldn't many (by no means all) pay a small fee as appreciation for services rendered.

The value of Facebook is that everyone else is on Facebook, so if they started charging they would lose a large part of that value overnight. Services that start free and get big don't have a high success rate of going paid (Salon.com rings a bell). No matter how valuable something is, if you've been giving it away previously there will be huge push back when you start asking for cash.

Charging for premium features is a much better idea since you don't lose tons of users overnight. I'm not sure if it will work for Twitter, I guess it all depends on what those features are.