(Post updated on January 8, 2021. See section at the end.)

The whole world is using WhatsApp to message each other. I often do too, because I want to inter-operate with the rest of the world.
However, WhatsApp belongs to Facebook.
Although Facebook has promised otherwise (fast-forward a few years: Yes, they did break that promise. See updates at the end of this post.), the temptation to link all of your WhatsApp messages with Facebook logins (a straight-forward process, as they have the mobile phone numbers of a great number of their users) must be quite tempting to the people at Facebook. Imagine how well they would then be able to target their advertising, based on their access to both your Facebook profile and your private WhatsApp messages!