Alexa is a way to have a general idea how big the website is, it depends though on the Target of the website, WJ has a rank of 2,112 Globally but because its a webmaster community a ranking of 2,112 (and many have Alexa plugin) i give it an estimate of having around 30 - 50k daily visitors.
But if a website with a 2,112 ranking that is not related to technology or things that require advance knowledge on computer it might mean that the website has 90k visitors a day because much less have the alexa plugin.
There is nothing useful from Alexa ranking expect for this point.... (it will give credibility for your website if you have a ranking less than 10k).
Experts see Alexa ranking as a valuable resource, to estimate the size of a website with taking in consideration other variables, if a website is less than 1200 it should have about 250,000 visitors and more (again depending on the target audience and the pageviews...etc) ....
for example i have a website that goes through it 1,300,000 users a day, but because they don't browse it and usually have 1 - 2 pageviews the ranking of the website is only 1100.