Private messaging app takes internet by storm...

Threema: Costs $3/£3. Audited source code. Based in Switzerland. Anonymous sign-up.

Threema: Costs $3/£3. Audited source code. Based in Switzerland. Anonymous sign-up.

Threema - Secure and Private Messenger
Threema is the messenger that puts security and privacy first. No other chat service offers a similar level of security and privacy.

Looks quite good according to ProtonMail's Blog:

7 WhatsApp alternatives that won’t share your data with Facebook
We take a look at some of the best open source and end-to-end encrypted alternatives to the WhatsApp messaging app.

(PS. Trying out clickbait titles ;-) )