http://t.me/memo_card_bot/app
(source: https://github.com/kubk/memo-card/commit/340ce60cebeadc32397c71f5ccd341129764bbc6)

A flashcard Mini App that allows you to improve your memory through spaced repetition. Learn languages, history, or other subjects with a scientifically proven method.

Telegram is a perfect platform for implementing such an app. Cross-platform, easy to share with your contacts, built-in push notifications (the feature is being worked on). Here is a sneak peek of the app: