I think the moany mums are missing the point. It isn't "boasting" or "proving" anything. It's letting your extended family see what your family including the kids is up to.
Yes! We live quite a way away from our F&F so I use FB as a way to keep people updated on our lives, what the kids are up to, their progress, things they do, etc, etc.
These people are our close friends and family, so they do want to hear about this stuff, we're friends/family - I like to hear about what their kids are up to, etc, etc. I'm interested in their lives...they wouldn't be friends otherwise.
But then I'm antisocial on FB, I don't have hundreds of people on there I hardly know or have no interest in hearing from.
I also ring them, email them, visit them, etc.