I would usually let the insults slide. People who are using social media to air a political grievance are not usually looking for solutions, or even reasonable conversation – they are treating it as a space to give vent to their anger. And that’s OK, we all do that. If you think you can change somebody’s mind, or help them out with a situation they think will never be resolved, invite them to email or message you – take the conversation into a forum where you can address each other humanly and civilly, and you will have more hope of making progress.
If it spills over into anything that makes you feel threatened, ask for the help that you need to stay safe. If posts on social media look like hate speech or abuse, report them to the site as well, and ask others to do the same.
Defending yourself on social media is an almost impossible task, but you can do some good by defending others. If you see someone else being misrepresented or attacked, step in on their behalf if you feel able to. I don’t know what it is about human psychology, but somehow a third party speaking up on behalf of someone is much more credible than that person speaking out in their own defence.