The Pew Internet and American Life Project is a good online resource. To find out more, you can browse their reports and charts (focused primarily on the demographics and online activities of internet users).
There are also some interesting statistics on the Nielsen Wire blog.
Readers, can you suggest other resources?
For some more in depth study, including great stats, I recommend the new Beacon Press book, The Young and the Digital, by S. Craig Watkins. Details at http://www.beacon.org/productdetails.cfm?PC=2083.
“The Young and the Digital” sounds fascinating; thanks for the tip!
For some great data also have a look at the Common Knowledge Reports on Social Media.
Thanks for the suggestion, Tim.