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.

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.