Earlier quoted context omitted.
I think this is the most important to me. On Android, this is only the second app that is likely to have a large userbase and e2e encryption for chats. The fact that its opt-in is a non issue to me. Didn't know WhatsApp had default e2e encryption. Might install it and check it out. I imagine it sends some sort of personal information back to the facebook mothership, though.
WhatsApp doesn't just have e2e encryption by default; they actually hired (or just contracted?) Moxie to implement the signal protocol: Over the past year, we've been progressively rolling out Signal Protocol support for all WhatsApp communication across all WhatsApp clients. This includes chats, group chats, attachments, voice notes, and voice calls across Android, iPhone, Windows Phone, Nokia S40, Nokia S60, Blackb…
The client (Whatsapp) can obviously read the data on both sides - and perhaps sent it to Whatsapp servers with e2e as well, so the data is transferred safely to Whatsapp/Facebook, without other eavesdroppers being able to see it :-)
My point is, if you don't trust the company that created it, it doesn't matter what technology is uses.
So who trusts Whatsapp/Facebook with their private data...?