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Re:Ejabberd Massive Scalability: 1 Node – 2+ Million Concurr... by zwily on Wednesday January 20, 2016 @07:43PM (#10942587)
I believe they use erlang, but not ejabberd.
Re:Ejabberd Massive Scalability: 1 Node – 2+ Million Concurr... by tiffanyh on Wednesday January 20, 2016 @09:51PM (#10943041)
Whatsapp uses a modified version of ejabber. Ejabber is written in erlang.

http://highscalability.com/blog/2014/2/26/the-whatsapp-archi...

Re:Ejabberd Massive Scalability: 1 Node – 2+ Million Concurr... by strmpnk on Wednesday January 20, 2016 @07:46PM (#10942595)
They started with a version of ejabberd of some sort and migrated to more custom components over time IIRC. I can't find the link right now. (Keep in mind that ejabberd is written in Erlang.)