• by Mark Walsh  @markfwal, May 31, 2012

The use of Twitter in the U.S. has remained steady over the last year, with 15% of online adults using the microblogging service as of February 2012 compared to 13% in May 2011. The main exception to that trend has been young adults (18-24), nearly a third (31%) of who now use Twitter, up from 18% a year ago and 16% in late 2010.

The findings come from new data released Thursday by the Pew Research Center’s Internet & American Life Project.

Twitter use by those in their mid-20s to mid-40s largely leveled off in the last year after roughly doubling between late 2010 and mid 2011. The frequency of use, however, has continued to increase, with 8% of adults overall going on Twitter daily compared to 4% a year ago. One in five people 18-24 now use Twitter daily.

The Pew study also showed a close correlation between Twitter and having a smartphone. One in five smartphone owners are Twitter users, with 13% using the service on a typical day. By contrast, internet users who own more basic mobile phones are roughly half as likely to use Twitter overall (9% do so), and just 3% of these more basic phone owners are “typical day” users.

Twitter says it has 140 million active users worldwide. In the U.S. Twitter.com drew 40 million unique visitors in April, according to comScore.