We've told a few close friends and family already, but definitely waiting to make it public knowledge until after the NT scan (a week today for me!) so we know everything is okay. Already having one child in the family with special needs means DH & I are 100% certain we would terminate if there's something wrong with the baby that we can find out before it's born. Horrible to have to think about, but already living with that experience makes it an easy decision actually.
I've seen some people say that they think a facebook announcement is a bit tacky, but after we've told DH's other son that's what we'll be doing because he'll tell his mother straight away and she'll go spewing to all our mutual friends - so we want to get in first! Also I have a lot of family overseas, so it's also the easiest way to let them know.