According to the Harris Poll, a survey of 2,220 U.S. adults surveyed online between March 31 and April 1, 2009 by Harris Interactive, about 48% of adults have either a Facebook or MySpace account, and only 5% of Americans are using Twitter.
If you look at the numbers carefully, they are far from grim. 74% of those aged from 18-34 have either a Facebook or a MySpace account while only 24% of those 55 and older have an account on these services. This is to be expected; social networking was always more interesting to youngsters; with time, I expect to see the older population participate more, too.
Twitter's 5% penetration, on the other hand, is quite an achievement, given how new the service is, and its once-all-too-frequent downtime issues. Of course, the buzz can die down, and users might turn their attention to another service. Twitter's growth in the last couple of months, however, has been phenomenal; if it continues on the same trajectory, it'll become a real social media powerhouse.