There are four commonly used terms when discussing site traffic: page views, visits, unique visitors and time on page/site

Photo by stallioA visit comprises the activity on a site caused by single computer within an arbitrary time-out period. The industry standard is 30 minutes. A combination of the IP address and a cookie helps identify and tracks the usage. This identifying information is also used to track unique visitors.
There are dozens of technical issues that muddy the waters – proxy servers, RSS feeds, AJAX page refreshing, etc. It’s a confusing mess. For me, the only reliable metric is page views, and that’s only valid if the raw data has been scrubbed.
A growing number of people use NoScript as a security measure; the likely consequence is that more people will only allow javascript from the sites they’re interested in, defeating, by default, javascript as a method of third-party traffic analysis and user-monitoring.
Third Party Ranking Data
Alexa Rank:
I don’t know of any (respected) professional search marketer that uses Alexa Rank as a measurement of ranking success. Data is pulled using the Alexa toolbar a user must install for Alexa to gather data. The database of information about sites that includes statistics, related links and more can be found on Alexa’s Site Overview pages, Traffic Detail pages and Related Links pages. Alexa Traffic Rank – View and compare Alexa Ranking graphs
Quantcast Rank:
Not just traffic data but demographics of the traffic. After pixel-tagging your website, Quantcast turns your site’s activity into a mathematical model, revealing positive and negative affinity levels across selected groups.
Google AdPlanner Rank:
Google AdPlanner combines information from a variety of sources, such as aggregated Google search data, opt-in anonymous Google Analytics data, opt-in external consumer panel data, and other third-party market research. The data is aggregated over millions of users and powered by computer algorithms; it doesn’t contain personally-identifiable information.
Comscore
Measures worldwide consumer behavior through its proprietary panel design, patented data capture technology and online data retrieval network. Expensive and not really worth it having, I am speaking from first hand experience with their services.
Compete Rank:
Site profiles estimate how many people visit a web site from a diverse sample of people that is statistically normalized and projected to represent the size and demographic composition of the total active U.S. Internet population. Solutions include combined device-level data with user demographics and online behavioral insights to deliver a complete profile of technology usage.
Web stats
Statcounter – famous free web tracker
HiStats – Free, real time updated web stats service
Addfreestats – provide free website statistics
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