How can I deal with untrue things said about me on social media?

There is this guy that i just met a few weeks ago. We got along so well when I first met him. Then we met in person and after we met in person, he said bad things about me on social media that made me look bad like calling me toxic and a negative person when I am not and now he completely cut me out of his life because he thinks that I’m a toxic person when I’m not toxic at all. Why would he accuse me of being a toxic and negative person when he doesn’t know me that well?

Just cut him out of your life & disconnect on social media. Ignoring him shows that you are mature. Getting into it with him proves his point.