I use WhatsApp constantly with my (fully remote) Level 1 ESOL class. We have a class group that allows us to communicate and share both during and outside of class time. I use it to share screenshots of our class notes and Zoom annotations live throughout class. Students use it to submit pictures of their homework. Students also use it during class to share pictures of their writing. And, since I use the desktop version of WhatsApp, I am able to share my screen and process/respond to everything they’ve sent during class. Being fully remote with beginners, I rely heavily on WhatsApp to be able to see what students are producing and gauge their understanding. And, because most students already use WhatsApp to communicate with their families abroad, the tech learning challenge is not too steep.

WhatsApp!
https://www.whatsapp.com/ 
(Desktop version for teacher, phone app for students)