Personally I think the paedophile thing is a complete red herring - the crux of the matter, for me, is still that it's a very public way of sharing a photo.
If you show people a print, that's one thing.
But suppose, as I said earlier, you're in hiding from an abusive partner, and someone puts a pic of you and your DCs on their Facebook account? It's much more likely that your expartner might see them there and track you down, via the friend of a friend thing, or because you haven't locked down your FB account properly, than it is if you're just showing prints to your close family and relatives.
Or, (and this could apply to me) suppose you're applying for jobs, and you don't want prospective employers to know you have children until you get to interview, as you'd rather impress them first and then let them know you're a mum second. (And don't tell me people don't discriminate against mums, I'll laugh hollowly...)
So you don't put pics of your DCs on your FB account, but anyone plugging your name into Google (as employers do to check out candidates) immediately brings up pictures of you and your DCs on someone else's unsecured account?
Again, you might say, it's not very likely to happen, but the fact that it could illustrates the fundamental difference between showing people pictures and posting them on social networking sites. For me, they're too public, it's sharing too much information about myself with everyone who has an internet connection.