Hmm, I wonder if the justice system has any reason for why some kinds of evidence aren’t admissible?

Nah, they probably just want to let a few criminals off the hook for funsies.

Your sarcasm aside, there are obviously plenty of reasons why there are failings of the justice system and most people agree that the government isn't the sole arbiter of social accountability.

I'm not saying the justice system is perfect, but we've spent literally thousands of years refining it and people have fought and died for things like rules of evidence, the right to face your accusers, trial by jury, presumption of innocence etc, etc. These things are still not a reality in many places in the world.

To throw that out and say "Nah let's nuke this guy's life because an anonymous person sent us a screenshot that they claim is him doing something racist" is not an improvement. This is moving in the wrong direction.

Good thing there are multi-billion private companies out there that can pick up the slack and punish the real criminals.

Anything posted on social media would generally be admissible as evidence in a US criminal trial, assuming that it isn't shown to be fake or irrelevant. That applies even if the recording was created illegally by a private individual.