Ignoring all the sniping pointless comments arguments which should really be confined to personal messages, or even better, not entered into at all - I use FB and have done for ages. I use it as a way to be nosy see what former colleagues are up to, similarly to see my two sons' activities (Yes, we do talk, but they often post immediate pictures) and the like. I've had several jobs so it's good to stay in touch, albeit in a virtual world, because there wouldn't be time in the real world, especially as some are in other parts of the world.
In my previous job in education, we were told to be careful on FB, not befriending students, obviously, and to be careful what we posted ourselves. That can all be avoided by using the security settings. I've never been compromised, seen or posted anything I'd rather not have done.
It's like most things really, it's a tool, can be dangerous is mis-used, but is still useful if done properly.