Everything we write on social media is evidence.
Any law enforcement agency, any other administrative agency, even any private attorney, can subpoena the social media records if they are relevant to someone's claim or charges.
The US court system provides zero privacy for people wanting to remain anonymous. And in today's violent America, being anonymous is basic life insurance. You don't risk a post using your real name unless you are willing to pay lawyers to defend you in court if you make a mistake ... unless you have no assets to lose, then it doesn't matter.