Try Quantcast.com. It ranks websites in popularity by monthly number of visitors. For many sites, detailed demographic information about website users is available. Browse the list of rankings or search for a specific domain. Two or more sites can be added to a chart and compared.
Another good source is Alexa.com. It ranks websites within content categories (health, shopping, travel, etc) and includes some demographic information on visitors.
Both sites offer additional information for a fee, but we encourage you to contact a librarian for research help before deciding to pay.
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