I don't have Facebook anymore (haven't for over 5 years now) but if I did I wouldn't be 'announcing' as its not my style. As it was I found it weird enough telling people in real life I was pregnant and only did so when it was so physically obvious lol.
My DH has Facebook and DD is 10 months old and he has never ever mentioned her on Facebook or has any photos of her on there. I like it that way too. He's from overseas so he only told mates when he actually spoke to them so one guy he only told a few weeks ago as he hasn't spoken since she was born.
I wouldn't be offended to find out at any point in whatever way it transpired, on the grapevine, bumping into you, whatever.