Maybe not today (reasonbably). But, until recently there were many apps (and still possibly many are) using Django (python) and Ruby on Rails (Ruby) as their development stack for web apps. Atleast for Ruby on Rails, I know of some popular apps that were built using it like Airbnb, Github, Spotify, Twitch, Soundcloud etc.

No, you've misunderstood. OP said:

"only language for full stack of this app"

The sites you mentioned used JavaScript on the client side, so Python and Ruby were not used as the only language for the full stack of those sites.