Few organizations track Wikipedia usage. Pew has carried out a couple
of surveys of American adults in recent years, listed below:
2007
http://www.pewinternet.org/Reports/2007/Wikipedia-users.aspx 2007
"36% of online American adults consult Wikipedia"
Pew found that in America Wikipedia was more popular with wealthy
people, white people and English-speaking hispanics, men, adults under
30, college graduates and home broadband users (obviously some of
those factors correlate).
Please note that Pew doesn't survey under-18s.
Wikipedia was the most popular education and reference website by
almost an order of magnitude.
"Over 70% of the visits to Wikipedia in the week ending March 17 came
from search engines, according to Hitwise data."
But the web and the way people use it has continued to evolve.
2010
http://pewinternet.org/Reports/2011/Wikipedia.aspx "53 percent of
online Americans use Wikipedia"
'In the "scope of general online activities, using Wikipedia is more
popular than sending instant messages (done by 47 percent of Internet
users) or rating a product, service, or person (32 percent), but is
less popular than using social network sites (61 percent) or watching
videos on sites like YouTube (66 percent)."'