While the media may have discovered Twitter, more than half of all Americans aren’t using it. Findings of the Online The Harris Poll, conducted between March 31 and April 1, 2009, show that 51 percent of Americans don’t use Twitter or have a MySpace or Facebook account. Forty-eight percent of adults have either a MySpace or Facebook page, with 16 percent of adults updating their page at least once a day.

There are some substantial differences in who is and who isn't using these social networking sites, according to the report:

--74 percent of those aged 18 to 34 years old have a Facebook or MySpace account, but this sharply drops off the older one gets: Just 24 percent of those 55 and older have an account.

--When it comes to Twitter, 8 percent of 18- to 34-year-olds use it, 7 percent of those aged 35 to 44, 4 percent of those aged 45 to 54 and only 1 percent of those 55 and older Men and women use Twitter at the same levels (5 percent each), but women are more likely to have a Facebook or MySpace account (52 percent vs. 45 percent)

Two in five people with a high school degree or less have a Facebook or MySpace account, compared with 55 percent of those with some college and 52 percent of those with at least a college degree.

There has been some discussion about whether these social networking sites may, at some point, become a threat to search engines such as Yahoo! or Google. Right now, that doesn't seem to be the case, the study found, since 45 percent of adults believe the sites are popular, but they won't pose a real threat to the domination of search sites, while just one in 10 say they may become a threat. Forty-six percent of Americans are not at all sure. Even among the largest users of the social networking sites, 18- to 34-year-olds, an overwhelming 62 percent say these sites won’t pose a real threat to the search engines.