Recent high-profile media coverage suggests a large percentage of the US population participates in online social networking and microblogging, but over half of Americans (51%) do not use Twitter or participate in either of the two largest social networking sites - MySpace and Facebook - according to (pdf) a recent Harris Poll from Harris Interactive (via MarketingCharts).

The survey finds that just under half (48%) of US adults have either a MySpace or Facebook account, and only 16% update their page at least once a day.

Usage statistics are lower for Twitter, with only 5% of Americans saying they currently use it.

As expected, Harris reports there are substantial differences in who is and who isn't using these social networking sites:

• Three-fourths of those ages 18-34 (74%) have a Facebook or MySpace account, but this quickly drops off the older people get. Just one-quarter (24%) of those ages 55+ have an account.

• Tweeting is also slightly more prevalent among the young, but not by much; only 8% of 18-34-year-olds use Twitter, while 7% of those ages 35-44 use it, 4% of those ages 45-54 and only 1% of those ages 55+ use it.
• While men and women use Twitter at the same levels (5% each), women are more likely to have a Facebook or MySpace account (52% vs. 45%).
• There is an educational difference in usage of MySpace and Facebook. Two in five people with a high school degree or less (40%) have a Facebook or MySpace account, compared with 55% of those with some college and 52% of those with at least a college degree.

Americans remain uncertain about whether social networking sites will eventually expand and grow in popularity to the point where they threaten search giants Google and Yahoo. When asked if they thought social networking sites could pose a threat to the big search sites, more than four in five (45%) adults believe the Facebook and MySpace are popular, but they won't pose a real threat to the domination of search sites.

Just one in ten (9%) say they may become a threat, while fully 46% of Americans are not at all sure. Even among the largest users of the social networking sites, 18-34 year olds, three in five (62%) say they do not think the social networking sites will become a real threat to the search engines, Harris reported.

About the survey: This Harris Poll was conducted online within the US between March 31 and April 1, 2009 among 2,220 adults (ages 18+) who agreed to participate in the Harris Interactive panel.

Figures for age, sex, race/ethnicity, education, region and household income were weighted where necessary to bring them into line with their actual proportions in the population and the sample was adjusted to reflect propensity to be online. The survey did not gauge the use of other social networking applications.