somebody posted on Facebook about a specific injustice they heard about and declared they will unfriend anybody who doesn’t care enough to post about it.

Sounds like they are doing you a favor. The world can be a pretty horrible place, and horrible things are happening all the time. You could spend all of your time posting about all of them online and accomplish exactly fuck all. You'd do far more by focusing on the things around you which you can actually affect. Your local community almost certainly has problems which you could help with. Even if it's just showing up at a food bank and helping to serve food to the unhoused. There are concrete things you could be doing to make the world a little less terrible for the people around you.

Posting on FaceBook et. al. is just slacktavism at its worst. You me or anyone else posting something on social media does nothing to change the facts on the ground. Assuming you're in a modern democracy, you do have some power with your vote and the ability to organize and be involved in collective action (e.g. political groups). But, even there the things you post on your personal accounts isn't going to actually do anything.

So ya, I'd tell the person to fuck off with their virtual signaling, slacktavist bullshit. If they really cared that much, they would get off FaceBook and put their time into actual community service and political action. But, they are too lazy to do the hard work, and instead just want to feel good about themselves with their smug, online pronouncements with no substance behind them.

Also, get yourself off FaceBook, the site is nothing but a cesspit of advertisements and assholes shitting all over everything. Case and point, your rant.