The US has arrested many people for speech, and even made charges stick many time. A famous historical example is charging Eugene Debs (and many others) with sedition for opposing WWI and the draft. There was at least one case of being arrested for political social media posts, already linked in adjacent threads. Threats of violence or even sufficiently harsh language to cause fear-for-life can be a crime. "Revenge porn" and deepfakes have had laws passed curtailing them and prosecutions made. The US is certainly less restrictive of speech than other countries, but you're nowhere near entirely free to say or post anything you want.