Why don't you use WhatsApp?!

There are many things involved, I try to write this in a non-technical way. So maybe it sounds a little too basic.

I don't want everybody who I want to write to also have my phone number.

I don't like to give my whole adress book, and thereby exposing the private phone numbers of my friends, to some company.

I don't like that WhatsApp knows with whome I write (what) and at what time.
They know at what times you usually chat and which times you probably sleep..

It's not like there is a person sitting going through your text. It's already enough to automatically evaluate your metadata.
Creating social graphs from your friends lists. Knowing who connected to whom at what time. When did they probably first meet, and where. Sorting people into groups deducing their interests from context.

Reading list:
I am sure you can use Google yourself.

I recommend using XMPP, but it still has some drawback.
If you are not a very technical person, like most people, I would recommend Wire as a good messenger.