I told close friends and family both times after the 12 week scan.
With DC1, I think I put it as a FB status update at about 14-15 weeks once everyone was told.
With DC2, I'd miscarried a few months before so while we told people IRL after the 12 week scan, I didn't feel comfortable saying anything on FB until after 20 week scan.
Have 12 week scan with DC3 this week. If all is well I suspect I'll do similar to how I did with DC2.
Both times I didn't really harp on about it after the announcement until the birth.
I know people who announce as soon as they've had a positive test, people who update every week with "28/29/30 weeks today" and people who say nothing until a baby is seemingly just delivered by the stork! All not how I do it, but the world would be dull if we all did things the same way.
People spend too long worrying about how people perceive them on FB, or thinking people's behaviour is wrong because its not his they'd choose to do it. Just do as you think is appropriate and sod anyone who has a differing opinion! But equally don't ever get sniffy about how other people use FB to share news.