Happened to me a few weeks ago. Two weans born around 5am and by 6.30am, at least three members of my partner's family had posted congratulations with names and pics in some cases - not to us, but on their own walls, tagging us in the statuses. This was before I'd managed to tell any of my aunts, uncles, cousins and close friends.
I had to quickly put up a post myself about them so that if anyone logged on that morning, they would hopefully find out the news from me and not others.
Fair to say I wasn't all that chuffed. The most important event of your life and you've got folk breaking the news for you. Lovely...