Seperate contact groups?
Err... you can!
Familiarise yourself with the concept of "Friend Lists" and Custom privacy settings. Once you've got friend lists set up, it isn't too complicated to determine who sees what in your profile (so, for example, personal friends would see more than colleagues, since they're more likely to know your education history etc. anyway).
Unfortunately, the problems occur when when sending messages: the only way you can restrict messages to specific friend lists is to click on the padlock icon, choose Custom, and type the name(s) of the friend list(s) into the box.
It would be so much nicer if Facebook realised that people have different social circles (not just one big grab-bag one), and the custom privacy options could be integrated into the main box (Everyone, Friends of Friends, Friends, Only Me), and it would be nice if a default privacy level could be set for each type of post (e.g. posts from FarmVille only go to people in my "Farmers" friend list, so the rest don't get spammed with application messages).
But they'd probably think doing this would make it too complicated for newbies... :(