So they say it was a different driver or a different car. Or they give a fake Instagram handle. Or they say the post is faked.

If you see evidence of a crime online, you should go and find the perpetrator based on that, not hope that you randomly stop him and find evidence because they are stupid enough to give the right handle.

Also, do you think it's okay for the police to totally ransack your car, because you might carry drugs or a corpse and get "rightfully arrested" because of that? I'm pretty sure you agree this would be over the line, so I don't see why scouring your social media in the hopes of getting a conviction is okay. Especially since this is just a minor extension away from looking through your DMs, and then your WhatsApp chat history ...