• by April 30, 2008

Despite phenomenal growth, Twitter still hasn't reached mainstream adoption, according to Heather Hopkins, vice president, research, Hitwise Intelligence, who analyzed the service's Internet visitation rates in the U.S.

While visits to Twitter.com are up eightfold year-over-year, and have more than doubled in the past three months alone, she noted that the site still ranked as #439 among all social networks and forums last week.

"Twitter's size is notoriously difficult to measure, as there are so many access points (mobile phones in particular)," she wrote. "However, the website traffic data does give some idea of the rate of growth and also reveals that the service still hasn't reached mainstream adoption."

Hopkins also reported that Twitter.com's top sources of traffic last week were social networks and forums, email services and search engines, but that the importance of search engines to Twitter traffic has been declining over the past year.

This stresses the strength of repeat visits from its loyal user base versus new users who come from search, she said. But both categories are still growing rapidly.