I think you're referring to Zendesk. Insofar as Alexa's rankings are accurate (they aren't, but that's another matter) then Zendesk.com is indeed the 650th highest trafficked site on the internet and they are clearly getting a lot of usage. That's great for Zendesk, but it says very little about how they compare to other B2B companies. Just as a low Alexa rank (above a certain minimum threshold - say 500,000) tells you very little about how a given B2B company is doing.

Example:

Website A: CRM software for real estate agents that charges $2,000 per month. Alexa rank 120,000.

Website B: in the same business but gives their software away free, lets users host blogs on their site and advertises with popups served from the domain (a huge Alexa boost to lots of porn sites, ad networks and others) and as a result has an Alexa rank of 9,000.

If you were to do a post on the real estate CRM industry you'd probably dump on company A and praise company B - even though we know essentially nothing about either business. Website A could be doing millions per month in revenue and have 50+ employees and B could be one guy working nights and weekends and making $2,000 per month.

tl;dr apples-to-apples comparisons between B2B companies based on homepage traffic stats = fail.