Just two years ago Twitter was the hottest thing on the web. But in the past year U.S. traffic at Twitter.com, the site users visit to read and broadcast 140-character messages, has leveled off. Nearly half the people who have Twitter accounts are no longer active on the network, according to an ExactTarget report from January 2011.
Just how popular is Twitter? Tracking its growth is complicated in part because many people tweet via dashboards such as TweetDeck (making it hard for Twitter to aggregate the mass viewership that many advertisers covet). The company says it has more than 200 million registered accounts (Facebook has 600 million subscribers), but users are allowed multiple accounts. The company also trumpets that the service had 155 million tweets daily by the end of the first quarter, a jump of 41% over the prior quarter, but many tweets -- news headlines, for example -- are often churned out by computers, not humans visiting Twitter.com. One key measurement of Twitter's popularity, Twitter.com's traffic, indicates a disturbing trend: ComScore shows that growth of U.S. visitors to the site has leveled off more than a year after its massive spike upward in 2009.
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