Adding a social mechanics to my product

Hi. I am working on a vocabulary app. I would like to add a game that has a social element and leaderboards. My intentions are for people to learn vocabulary in a social way. I also want to increase retention by creating variable content.

What are some social mechanics that I should add? I could start with the mechanics and then build the game around the mechanics that have my desired benefits.

My app is below if interested. It is a product but it does not use Stripe.

https://play.google.com/store/apps/details?id=com.clearfaun.spencerdepas.vc