It seems that Twitter is becoming more of news feed than a social network, said Paul Judge, author of the report and chief research officer at Barracuda. And that raises questions about its growth potential, as well as how the Internet phenomenon will make money.

As of December 2009, only 21% of Twitter account holders were what Barracuda defines as "true users," meaning someone who has at least 10 followers, follows at least 10 people and has tweeted at least 10 times.

That indicates that most Twitter users "came online to follow their favorite celebrities, not to interact with their buddies the way they would on Facebook or MySpace," said Judge.

The follow-only trend exploded when celebrities helped push the microblogging site into the mainstream during a six-month period that Barracuda calls Twitter's "red carpet era."

From November 2008 to April 2009, several celebrities, including Ashton Kutcher, Oprah Winfrey and John Mayer, joined Twitter. And the site grew 21.2% in the month of April 2009 alone.

"The most famous people have already joined Twitter, so I don't think they'll see another growth spurt like that," Judge said.

So the question now, said Judge, is whether Twitter can get more of these followers to start tweeting themselves.

"The bottom line is, most of these people are getting online because Ashton asked them to," Judge said. "If those people do nothing after that, [Twitter's] growth can't hope to continue."