[volaski]

There are many cases where a social site shot itself on the foot and committed suicide by taking one bad move, such as Digg. What you're suggesting sounds like it will set a new record for a bad decision. Let's say Twitter does make that mistake and actually starts charging high end users. It is extremely naive to think most of these people will stick around and pay. Some will start leaving. Some new services will arise that claim to provide the service for free. Some of these "celebrities" will join forces to create a new "free" network (because that's what Web is supposed to be). Overall their userbase will decrease. And just as it grew exponentially after it reached critical mass, the reverse also applies, and at some point it will die at an exponential pace. It will probably end up looking something like MySpace.

[reitanqild]

Maybe let existing users continue like today and charge only for new users?

Personally I think twitters problem is much simpler: it is just a media/subset of biz/subset of tech thing.

The only thing twitter has over other services is brand recognition and a certain community.

On all other areas they are owned by both Facebook and even Google+

I remember being exited about twitter years ago: I saw lots of possibilities like "event streams" for automated processing, I thought they'd come up with a way to mark certain messages as mostly relevant for certain groups of followers etc.

What they did instead was focusing on painting themselves into the 140 revolutionary characters corner and keeping the paint wet from time to time by messing with the API etc.