This is exactly why I turned chat off. I have no desire to be contacted by old high school classmates who happen to be bored and browsing facebook, and see me pop up.
With the old chat design, choose chat at the bottom right, then the "Friend lists" menu item in the chat window, and add the friends list you want to affect chat availability, and then turn availability on/off to particular lists.
Useful for parents and work colleagues who don't use other IM or who you don't want to give your personal IM accounts to.
(Personally, I still feel surprised whenever I see anyone even using Facebook chat.)
I think its largest drawback was using account numbers, instead of allowing alphanumeric vanity names.
I don't know, was you the only person on that service and thus have no-one to talk to?
In short, Facebook likely contains everyone you've ever met or dealt with in person or online and "friended" that can potentially chat with you -- IM likely contains only the people you actually need to chat with.
"Everyone is there" is becoming an increasingly poor reason to justify selling your soul to a particular social network (as an individual or a business).
They've basically just brought facebook chat on par with instant messengers from the 2000s except you have to install something additional for video chat.