The title says it all. What programming languages will I need to learn to create a social media app/website like Facebook?
Facebook is written in PHP. I don't think that anybody would recommend creating a site in PHP these days. Facebook even needed to create their HHVM to improve performance and compensate for PHP's shortcomings.
In all honesty, there are a million ways you could go. Myself, I would recommend Spring Boot for the backend and React JS for the front end, but that is just an opinion.