The Pew Research Center's Internet & American Life Project has released Wikipedia, Past and Present.
Here's an excerpt from the announcement:
The percentage of all American adults who use Wikipedia to look for information has increased from 25% in February 2007 to 42% in May 2010. This translates to 53% of adult internet users.
Education level continues to be the strongest predictor of Wikipedia use. The collaborative encyclopedia is most popular among internet users with at least a college degree, 69% of whom use the site. Broadband use remains another predictor, as 59% of those with home broadband use the service, compared with 26% of those who connect to the internet through dial-up. Additionally, Wikipedia is generally more popular among those with annual household incomes of at least $50,000, as well as with young adults: 62% of internet users under the age of 30 using the service, compared with only 33% of internet users age 65 and older.
In the scope of general online activities, using Wikipedia is more popular than sending instant messages (done by 47% of internet users) or rating a product, service, or person (32%), but is less popular than using social network sites (61%) or watching videos on sites like YouTube (66%).