One reliable source for the number of Twitter users is Twitdir. TwitDir is a Twitter users (some call them twitter'ers) directory.
Twitdir reports the number of Twitter users is well over one million.. As Twitdir is only capable of detecting Twitter users with a public profile, this number is a underestimated as to the total number of Twitter users.
According to a story in TechCrunch entitled, "End Of Speculation: The Real Twitter Usage Numbers." the key measure of Twitter usage is total users, total active users and total messages sent. And according to a source close to the company, these are the current Twitter usage stats:
Total Users: 1+ million
Total Active Users: 200,000 per week
Total Twitter Messages: 3 million/day
SocialMarketingExpert.org states that since Hitwise says web visits to Twitter have increased 8x in the last year, they estimates the number to be 14,590,000 Twitter accounts in total (most of which may well be "inactive")-- but around 3 million active accounts.
According to figures from eMarketer, Twitter receives over 14million visitors a month from the USA and they estimate that Twitter currently has over 6 million users.
Millions. If you visit their website there is a graph of how many people are using it.
Twitter crossed 200 million members in March,2011.
I have been stalked on twitter but most people aren't.
190,000,000 people follow Twitter.
There aren't any statistics to show exactly how many people of a country use Twitter.
There are many. Find out for yourself by searching for their name on twitter.
This is unknown, try asking the Twitter Developers! :)
As of January 2012, over 300 million people use twitter.
As of March 17,2011 8,202,406 people are following him on twitter
As many as you want.
Twitter reached 200 million members in March 2011.
Many famous people use Twitter. Lil Wayne's Twitter account is @liltunechi