The number of Twitter users reached an estimated 75 million by the end of 2009, up from approximately 5 million in the previous year, but the growth rate of new user accounts is slowing and only 20% of Twitter users are active, according to a study from RJMetrics.

During the last six months of 2009, Twitter registered a steady decline in new user accounts: The growth of new users peaked last July at an estimated 7.8 million new users a month, but dropped to 6.2 million by December, down 20% from its high.

Twitter Account Activity

At the end of 2009, a large portion of Twitter accounts were inactive:

• 25% of registered accounts had no followers.
• 40% of accounts had never sent a tweet.
• 80% of Twitter users had tweeted fewer than 10 times, up from 75% in August.

In addition, only 17% of Twitter users sent a tweet in December, an all-time low.

Average Number of Followers Down

Twitter users had an average of 27 followers at the end of the year, down from 42 followers in August.

Moreover, the portion of Twitter users who have no followers increased to 25%, up five percentage points from the 20% recorded in August.

A likely reason for the drop in the average number of followers is Twitter's rapid growth rate—one-third of Twitter's user base has joined up in the past four months—which pushes the average rate of followers downward.

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Younger Users Are Fastest-Growing Demographic

Twitter's user population is getting younger: By the end of 2009, approximately 34% of Twitter users were under age 25, up 14.1 percentage points from the 20% recorded in 2008, according to comScore.

The initial success of Twitter was largely driven by users age 25-54, who comprised 65% of all visitors to the site in December 2008, with those under age 18 accounting for 11%, and those age 18-24 accounting for just 9%.

Despite Twitter's initially older skew, as it gained widespread popularity with the help of celebrity Tweeters and mainstream media coverage, younger users flooded to the site in large numbers during the year, with those under age 18 (up 6.2 percentage points) and those age 18-24 (up 7.9 percentage points) the fastest-growing demographic segments.

About the data: Twitter findings were compiled by RJMetrics in December 2009 and are based on Twitter application programming interface (API) data.