I've never used FB but I did use Google+ for a while, not really for the social aspects but instead to join Android app betas, this was before Google moved the beta signups to the Play Store, and I always found the way G+ allowed you to define different circles to be a very nice feature.

I know G+ gets a lot of hate but the circles aspect seemed well implemented and made compartmentalizing your different social/professional circles much easier and more transparent. I wonder why FB doesn't allow something similar?

I've been tempted to join FB in the past, not to partake in the social activities, but as a tool to manage my contacts, but I'm incredibly uncomfortable with the idea of my different social groups and my professional contacts sharing the same connections to me. If I could completely separate different circles to ensure they have absolutely no overlap or ability to see my other connections I would probably use FB.