I agree that closed source is harder to verify, but that’s not the same as insecure.
Personally I use Telegram because they don't mind non-official clients like the FOSS builds. Whatsapp on the other hand is known to ban non-official clients. API access isn't available either unless you are a big corporate user, at which point you need a facebook ads account and cooperate with some API reseller... no joke lol.
They do have access to the metadata, i.e. whom you messaged and when.
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[0] https://techcrunch.com/2016/04/05/whatsapp-completes-end-to-...
I love Signal and all the work Moxie and his team have put into it and the protocol, so this isn't a diss to them, but just wondering what the disadvantages would be for someone just looking for an E2EE communication app.
One difference I suppose would be that Facebook would have all the message metadata; just not the contents.
Whatsapp still does E2E encryption. (Same as Signal) Facebook can't read your messages. (There's still the scenario of you getting a special version of the app with that functionality disabled of course) You can also enable notification for changed signatures, which I do see changing as people replace their phones.