IN Social Media or ON Social Media? Which is correct?

The correct preposition, in this case, is “on social media”. We use “on” for anything that is used on a screen such as Facebook, websites, YouTube, etc.

I haven’t been on social media in weeks. 

I want to check him out on Instagram. 

You can read my full guide on how to use prepositions of place like on, at, and in here.