The study found that in 2008, 6 million U.S. adults - representing 3.8 percent of all adult internet users - accessed Twitter at least monthly. That number is expected to grow 200 percent to reach 18 million, or 11.1 percent of internet users, by the end of 2009.

Twitter's growth will not slow after this year, either - eMarketer projected that by 2010, 26 million U.S. adults will be regular Twitter users, representing 15.5 percent of all American internet users.

The 2009 figure has been upwardly revised from an April eMarketer study, which projected that there would be 12.1 million U.S. adult Twitter users in 2009.

As evidenced by this growth - and the attention given to social media in recent news - for small business owners Twitter represents a valuable resource in a number of areas, from marketing to customer relationship management.

A recent study from iPressroom, Korn/Ferry International, the Public Relations Society of America and Trendstream found that small- and medium-sized businesses were more active on Twitter than their big business counterparts, at 64 percent SMB adoption compared to 47 percent for larger businesses.