The internet is a great time-waster. Facebook even more so.
Ostensibly a networking site, its best value is as another tool to allow people to keep in touch with each other despite the inevitable tyrannies of time and distance. While snail mail can be personal, mobile phone direct and email easy, Facebook is a permanent placeholder that can be as low maintenance as desired, or generate a constant stream of traffic between people or groups. Yet the bottom line is that one can easily check up on what a friend is up to - without necessarily having to engage. And you never have to lose contact. In fact, solely through Facebook, I've restored a number of contacts I thought were lost for good.
But you get all types. Facebook is also good for communicating interests. But some people almost live on that site, sending/receiving a steady stream of actionable items - questionnaires, games to play, and groups to join (thanks, Andrew, for all the worthy causes - and they are all good, especially the group declaring Steve Fielding to be not real - but there's only so much time in the day...)
I also get a number of friend-linking requests from people I don't know - largely friends of friends, I guess, with a smattering of mistaken identities. I ignore anyone I don't know well. It keeps the names down to a manageable number. Not like some who collect names with little discrimination, and get lost in the morass.
Recently I saw a (Wiley) cartoon - quite funny, but sobering - that typifies for me that rush of blood to the head that accompanies Facebook, and the lack of discrimination that dilutes its use as a tool.
LinkedIn is the equivalent for professional communication. The equivalent collector there is the professional recruiter - many are casting wide their search for names with apparently little planning on how to use LinkedIn as an effective business tool. I accepted a few invitations to link before I realised I was going to get swamped. So with due respect to recruiters (as a contractor, I'm constantly looking for work), I no longer accept invitations from them.