My wife will post something about our kids on Facebook (like a couple of photos from an air show we went to last weekend) for her family to enjoy. It beats e-mailing all of them about it, or calling them all to let them know we went to an air show, or dropping in on her sister in Maine or her other sister in Indiana to tell them about the air show. Sometimes a couple of photos and a quick comment are all that loved ones need to connect and feel like they're in the loop about what's going on with our family. It's insane to think that every little event deserves a phone call, but it's perfectly reasonable, IMO, to put that stuff out there for family to enjoy at their own leisure.
It's really as simple as that, despite the over-the-top stuff that folks try to interject in here. Facebook operates within a greater social context, not outside it.