Ah thanks for the precision. I'm not exactly sure to be honest of the lineage of whatsapps/mongoose, some sources says it's based from Mongoose and/or had involvement from Erlang Solutions that ended up in Mongoose.
Anyway Mongoose and eJabberd have an amazing lineage and track record of uses and forks :)
I joined WhatsApp in late 2011 and left in late 2019. We always said we started with ejabberd and then replaced everything over time to get our system. I'm not sure about engagements with Erlang Solutions. I think that may have predated me, or it may have just not included me as I never did much with our chat server.
Facebook's chat system was built up with the same pattern: start with ejabberd, replace what you need to, eventually it's not the same at all (FB rewrote in C? because they couldn't hire Erlang people. WhatsApp rewrote in Erlang because Erlang is a very good fit for a chat app, and hired non-Erlang people and gave them Erlang books). I think RiotGames started with ejabberd as well.
Mongoose IM is a fork of ejabberd: - https://www.ejabberd.im/ - https://github.com/processone/ejabberd