Twitter usage numbers - My bet is 350 - 400K users

Twitter's upcoming next round of financing has recently revived the guess-how-many-registered-user-Twitter-has game, earlier today culminating in some new bets published by Michael Arrington, and by a bunch of commenters, under the promising TechCrunch title "End Of Speculation: The Real Twitter Usage Numbers".

I think all their guesswork (from 1+ million to nearly 4 million; elsewhere I've even seen figures above 10 million circulate) is way too positive.

When I simply (a) use Twitter's own location search as a basic input, then (b) taking the Top 50 of cities in the world where Web 2.0 things are most popular, then (c) counting how many registered Twitter users have specified any city in their profile (75%), and finally (d) multiplying that by a factor of 100/28 (28% is the share of the Top 50 cities in the world's Web 2.0 population in the benchmark stats that I used), the result is 375 thousand users.
I've doublechecked this with a similar extrapolation where I took the Top 50 of Web 2.0 countries as an alternative for (b), and the percentage of users specifying their country (40%) as an alternative for (c) - etcetera. The result is 350 thousand users. Close enough.

I did both exercises in a quick & dirty mode, using the stats of quite a different social media company as a benchmark. But I promise to eat my shoe if I'm more than 25% off the real, actual Twitter user figures.

For a social app like Twitter - with its strong connection to the real world - to reach the tipping point in any civilized market, the first thing you need is local density, i.e. a penetration of at least 1 to 2% in the biggest cities' populations. Twitter is nowhere near that level in any of the 50 cities that I've checked.
If I were them, I'd spend at least half of the next round on facilitating & promoting local events, meetings and parties in all major cities in their Top 12 markets, thus strengthening the ties between their users in real life, and boosting their density to the right levels.