In many ways you can differentiate your friends on facebook.
When it comes to social networking sites, I have had them all... and facebook is the best of the bunch.
If you don't know someone very well... the solution is simple. Don't add them.
There is a tool that allows you to write "how you know" somebody... and you can also put friends into groups.. ie friends at Queensland University of Technology or create a "Top Friends" to show who your closest friends are.
Just thought I'd make those few points.