TeenVogue - yes, you read that right - have written a guide to secure messaging, and it's excellent.
Short version: Update & secure your phone.
If you don't want anyone reading your private messages, don't use SMS, use WhatsApp.
If you don't want Facebook knowing who you message, when & how often, don't use WhatsApp, use Signal.
I know by reputation 2 of the 3 experts quoted in the article and they are 100% sound. It's in plain english. Read it.
http://www.teenvogue.com/story/how-t...essages-secure
(FB use WhatsApp data to connect people and "improve" their suggestions for "people you might know". FB has outed both clients & providers to employers and spouses in the past. In the security model, FB is a Threat. If you have anything to lose, treat them like one.)