By RPS, I imagined Shopify is largest. But then from what I understand, Shopify operate in such a way that every shop is its own app. i.e there is 100s of thousands of Basecamp / AirBnb running on the same code base and every shop is somewhat isolated.
Purely in terms of App in think the largest would be Cookpad. AirBnB never shared their numbers so I don't know.
https://speakerdeck.com/a_matsuda/the-recipe-for-the-worlds-...