Turnitin is used at Empire State College to check assignments for plagiarism. They recently conducted a study to check what kinds of websites students commonly use in their written assignments (at all schools, not just Empire State College). They found some interesting patterns:
One-third of their findings matched information found on social networking sites (like Facebook) or content sharing or question-and-answer sites (like Yahoo Answers).
Fifteen percent of their plagiarism matches came from cheat sites. You can find our more about cheat sites and what to avoid at http://www.sunywcc.edu/dep/english/plag/cheatsite.htm
25% of their plagiarism results came from legitimate educational websites, many more than come from social or cheat sites.
The most popular website used in plagiarized work is Wikipedia. The other most common websites used were answers.yahoo.com, answers.com, slideshare.net, oppapers.com, scribd.com, coursehero.com and medlibrary.org.
For more information or to view the study and their findings, you can read it at this link that will go directly to the PDF file of the article.