I've never seen a Facebook friend announce a pregnancy and thought it was silly or attention seeking.
At my very least positive, it would be someone maybe from school I'd forgotten I was connected to appearing on my page as their post got a lot of likes... And I would think "Oh, so-and-so is having a baby," usually followed by "How nice, best of luck to her/them."
Much more commonly, it would be an old friend I'm not in frequent contact with anymore but still have very fond feelings for. When I see those annoucements, I always feel such joy for the person! It doesn't matter that we're not close enough for me to have found out in person!
I can't imagine feeling negative, or anything worse than neutral about an announcement on Facebook.
That said, I don't know how to advise you as this is my first pregnancy and I'm not sure yet what I'll do or if I'll do a Facebook annoucement at all (I don't use it freqeuntly) or, if I decide to go ahead, when I'll do it. I just wanted to add another bit of support to say that if you do an announcement, well I'd be happy for you if it popped up on my feed ❤️