Traffic rankings and visitors are generally available for main domain names. To check traffic and visitors stats for sub-domain names various available free tools on web can be used. Some tools ask you to add a code on your website and other just gives the data based on visitors patterns.
Google Analytics can be used to track visitor traffic along with other data and stats for sub-domains. To track pageviews and reach one can use http://www.who.is/website-information/DOMAINNAME.TLD/ also.