From the Chicago Manual of Style's Quick Guide (http://www.chicagomanualofstyle.org/tools_citationguide.html)
A citation to website content can often be limited to a mention in the text (“As of July 19, 2008, the McDonald’s Corporation listed on its website . . .”). If a more formal citation is desired, it may be styled as in the examples below. Because such content is subject to change, include an access date or, if available, a date that the site was last modified. In the absence of a date of publication, use the access date or last-modified date as the basis of the citation.
McDonald’s Corporation. “McDonald’s Happy Meal Toy Safety Facts.” Accessed July 19, 2008. http://www.mcdonalds.com/corp/about/factsheets.html.