TheLadders was present at the NYC@Boston startup event a few weeks ago, which supposedly had to turn away some companies interested in being there. I'm surprised the people running the event would let a company like this recruit.
A Lisp/functional programming meetup group I sometimes attend is hosted by The Ladders. Their offices are actually really nice and look like it'd be a great place to work for a developer. It's awesome of them to let us use their space and equipment; I know nothing of their business practices, however.
I'd say they'd need a lot of people answering phones, a good couple lawyers (looks like they'll be expanding their legal team soon), lots of marketing and sales folks (churn must be a problem) and a couple programmers. I'd be surprised if it takes more than half-a-dozen to write page-scrapping robots and keep a job board site.
OTOH, as was pointed out earlier, it looks like a chaotic, noisy environment. If you have that kind of environment, you may need to multiply your team size by 1.5, at least.
edit: from http://careers.theladders.com/ it looks like they are using Java, Spring and are fully enterprisey buzzword compliant (only missing an Oracle RDBMS). If that's the case, you can multiply your team size again by a factor of 3.
edit 2: talking with a colleague from another company (they do a lot of Ruby stuff) I could confirm that our two-week sprints (we do mostly Java in this cell) are too long for them. Your iteration time varies according to the tools you use.