“Arbitrary” is a bit harsh, but it’s definitely close. There are a lot of services that attempt to rank web pages online. We’ve included a few of them in this section for comparative purposes, which is mostly the only thing they’re good for.
Pagerank used to be the only thing anyone in search marketing cared about. Now it’s not that big of a deal. Pagerank is one of over 100 factors Google takes into account when placing links onto a search engine result page (SERP). Pagerank is a good indicator of overall SEO/SEM clout, but don’t get too hung up on it. You should also note that it’s a logarithmic scale. Just like the Voltmeter overall score, it’s not twice as hard to score a two as it is to score a one, it’s 10 times harder; and scoring a three is 100 times harder and so on…
Alexa has been around, in some form or another, since the beginning of the web. They’re a “web information company” that gathers said info about said web from people with the Alexa toolbar installed on their browsers. Then they try and extrapolate the rank of every website online and list them on their site, among other things. It’s a lofty goal and they often fall short, but overall it’s one of the most reliable services of its kind.
Compete.com does pretty much the exact same thing as Alexa. They try and gather all the information they can about the net and then package it for their users/customers. Also just like Alexa, these numbers don’t mean much on their own and are mostly good for making comparisons between two sites that are both ranked by the service, since they’re both ranked using the same data set.