WhatsApp or not
I am not using WhatsApp. I also do not have a Facebook account. For Facebook it is easy: I'm just not into sharing my personal life online and keeping in touch with friends can be done without it. WhatsApp is somewhat different. It's basically a replacement of text messages on steroids because you can have groups.
My primary reasons for not using it are: encryption, sharing my phone number with Facebook and open source. For this reason I was and still am promoting Signal. The code is available for anyone to read and audit and cryptography scientists have reviewed it. You can run your own server so you don't rely on any organization to run the server for you.
With the latest news of WhatsApp adding encryption by default and using the Signal protocol this argument is void. So it boils down to the one question: do I trust Facebook with my data? I have no way to check their code and see if there is a backdoor that circumvent the encryption.
But here is the thing: Facebook already has my data. Since WhatsApp downloads the address book of all my friends that have it installed, they already know my phone number. They can even guess what my social network roughly looks like.
So I'm doomed. And I'm asking myself: do I keep being the one person in the room not using WhatsApp?