Can't really add anything to RoverDom's excellent post, other than it has helped me to reconnect with ex football team mates, school friends and even extended family in a really positive way. The flip side of the same coin is that you do occasionally get people coming back into your life - in a virtual sense anyway - that you haven't seen in 30 years for a reason!
I used Twitter a lot in my previous job, not so much these days, and also LinkedIn has proved useful on a couple of occasions, but its mainly BRFCS and Facebook for me.