Actually, we do know when you are on the Facebook corp network and can filter by that, but we rarely gate on that. We have a system we call gatekeeper for controlling the launch of new features. If I'm coding up something new I'll usually just add my own user ID (we call GUIs in Facebook FBIDs) first. The gatekeeper system is a very full featured roll-out system. I can launch to just employees, 1% of users world wide, Facebook users in Peru, viral growth mechanism, etc..

We also maintain a robust employee list that is cached in APC on every web host that you can always call an is_employee style function for any user ID on. The careers site in particular has some employee only functionality that this endpoint is probably checking.