This isn't bad advise if the wording wasn't so robotic.

You can empathise with someone for being upset without contributing to further the discussion. In situations like this I normally redirect... So if I received a message like, "I'm really mad at X-Corp and Mr Bossman, I can't believe they got rid of Joe!", I'd probably reply, "it's sad, I hope he's doing alright".

As long as you're not saying "yeah, f** Bossman!" you're probably fine.

That's obviously assuming you want to avoid trashing your employer... I find these events are often times for reflection. It's okay to be critical if you feel strongly, but you should do that in a professional and productive way. But if something happens which you strongly disagree with then you have a responsibility to do what you think is right, sometimes at personal cost.