I think some people are being deliberately reductive on this thread. It obviously matters who/how many fb friends you have and how close to them you are. It's very different if you only have close friends, but that's not the case for the majority of people in terms of how they use Facebook.
You don't have to feel that everyone is "out to piss you off" for a surprise scan photo to catch you off guard at a low moment and upset you.
For those saying that you should defriend People or not go on social media if you have previously suffered a pregnancy loss, I don't think that's particularly realistic. The point with pregnancy announcements is that they are usually a surprise, and once you've seen it it's too late.
Sure, the risk of possibly upsetting someone you're not close to doesn't have to influence your decision to do it, but you should be aware that you might.
I think, though willing to be corrected, that people make these announcements because it's lovely news and you want everyone who sees it to press like and comment and congratulate you (ie you're doing it for yourself), which is fine and completely understandable.
But you need to admit that and don't pretend you're doing some kind of public service broadcast, as mostly the majority of people who see it don't need to know and won't especially care, and you should also acknowledge that some people might not be as happy to see it as you would hope, whether they'd say that to your face or not.