Sure you can but alexa ranking is only counted for traffic that has alexa mechanism associated with it.
Alexa is great, but sometimes does not track lower ranked sites. Sometimes I find my self using: siteanalytics.compete.com
I always check a sites alexa and other rankings before I buy advertising, so IMO I believe that it is a somewhat good way to see how popular a website is. Usually a decent ranking is less than 500,000.