Once considered a social media curiosity, Twitter has made inroads in government just as it has become a mainstream tool for millions of people worldwide.

GovTwit.com, a Web site that tracks Twitter users in the public sector, reported that its master list of Twitter user names affiliated with state, local and federal government has increased to approximately 2,100 -- more than a tenfold increase during the last eight months.

"Government agencies and leaders worldwide have flocked to Twitter as a critical part of their citizen communications efforts," Steve Lunceford, the creator of the GovTwit directory, said in a press release.

Early adopters included the U.S. Environmental Protection Agency and the Defense Department, though more government workers are tweeting under their own names instead of through an account that's attached to an agency. This includes governors -- California Gov. Arnold Schwarzenegger tweeted Tuesday about signing the state's budget -- and IT officials like Dave Fletcher, the chief technology officer of Utah.

Twitter is a free-to-use "microblogging" Web site. Messages, called "tweets," may contain no more than 140 characters. They are commonly sent and received from mobile devices or PCs via a traditional Web browser or text messaging. Users can subscribe to any other person's updates, which creates Twitter's interconnected web of "friends" and "followers."

Twitter doesn't disclose an official tally of its user base. Analysts have speculated that there are anywhere between 2.5 million to 10 million accounts worldwide.

In recent months, Twitter's security has come under scrutiny after several celebrities' accounts and passwords were hacked. Last week the United Kingdom's government released a 20-page guidance document intended for departments that are interested in using Twitter. The guidance said strong passwords for logging on to Twitter should be changed frequently and should be kept by no more than two members of an agency's digital media team.