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Signal and Threema want nothing to do with WhatsApp
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
If it's an optional feature why are you complaining that the other businesses are refusing their option to federate with Facebook?
The issue is simple: Facebook will work to leech users away from other services, strengthening their position into a monopoly (if it isn't already in some places). It is not a good thing for Facebook to get access to more users and steal their data.