Sorry to keep you waiting. I finally got my answer. It's not very encouraging, but it should be doable.
There is no official way to join the game on Facebook. That requires a browser and the game officially doesn't work in browsers anymore.
However, there is an unsupported way to use one: you need to use Puffin browser to be able to start your account on Facebook, which you do here. Your friends will still have to tell you which map they're on.
Alternatively, you can keep playing using Puffin, but it's a paid-for browser, and the gift codes we sometimes get are not supported when playing outside of Plarium Play.