Fans haven't heard a tweet from these celebrities in months.
Miley Cyrus has quit Twitter. After tweeting up a storm for months, last fall the 17-year-old pop singer posted a message alerting the world that she and her boyfriend, actor Liam Hemsworth, were no longer fans of the microblogging service. "FYI," she told her 2 million followers, "Liam doesn't have a Twitter and he wants ME to delete mine with good reason." And just like that, she was off.
But not for long. Almost immediately, Cyrus posted a video of herself on YouTube, telling fans exactly why she was quitting Twitter. "I want my private life private," she intoned. "I'm living for me." She did not address the irony of releasing a video on the Internet expressing a desire for privacy.
High-profile defections pose a problem for Twitter, because they reflect an unpleasant truth about the site: that most people who join Twitter barely use it at all, and many never return after the first few logins.
According to a study published in the Harvard Business Review, the median number of lifetime tweets per user is one, and over half of the estimated 20 million people on Twitter tweet less than once every 74 days. That means that the vast majority of conversation on the site is produced by a very small group of people; the same study found that 10 per cent of tweeters make over 90 per cent of all tweets. A separate study by Nielsen found that more than 60 per cent of US - based tweeters ditch the site after their first few posts.
This is not to say Twitter hasn't grown. According to Nielsen, between February and August 2009, Twitter users jumped fivefold from 5 million to 25 million, although membership has since flattened to roughly 20 million. But the site continues to struggle with engagement.
"Month over month, the fact remains that the majority of Twitter users are not going to be around the following month," says David Martin, the primary researcher at Nielsen. "In order to maintain growth, you have to continue to retain a large number of users."
In a statement to Forbes, Twitter said it is working "to make it easier to find interesting content, improving search capabilities, developing geo-location services and working with thousands of third-party developers to create awesome experiences off of Twitter.com."
Still, the essence of Twitter's retention dilemma remains: What's with the Houdini act? The list of high-profile Twitter quitters offer various clues. Some, like Cyrus, leave for reasons of privacy. Others don't see the point. But most are just plain too busy to tweet.
Certainly no one can blame Twitter for the departure of US Vice President Joe Biden, who last tweeted in August of 2008. He did have a country to run. But disgraced former US Senator John Edwards? Surely he's had the time to post - but he hasn't since November of 2007. The US Republican party has its share of Twitter quitters, too: Oklahoma Senator James Inhofe last tweeted in December 2008. Florida Senator Mel Martinez hasn't made a peep since last July.
Several top comedians appear no longer amused by Twitter. Stephen Colbert bid adieu to nearly 400,000 followers last August: "I've taken a long vacation. All I do is say funny things. And I'm thinking of raising my own salary. What am I a senator?" Ricky Gervais failed to see any purpose: "I am sorry, but I am going to stop these tweets because I don't see the point," he wrote. Larry David deleted his account entirely in January 2009, but his last posts helped explain why: "So, I've been sitting here for 10 minutes. No one has said anything. Is this supposed to do something? Did I break The Twitter?"
Other celebrities may have given up the service because of outside criticism. Musician Dave Matthews was an avid tweeter, often posting 20 times a day. But when Rolling Stone wrote an article about rock stars on Twitter and mocked his childish tone, the end drew near. His fans haven't heard a tweet since October.