Threema. Self funded, no investors. Hosted in Switzerland on own hardware. Focus is on maximizing privacy and not having/storing any customer data. (Disclaimer: I'm an employee/developer.)

Not OP, but: Check out Signal and Telegram for example.

or even Matrix/Riot

I'm using Matrix/Riot to communicate with two of my friends. It's really nice in general. But if you want to use E2E encryption, the key exchange is really tedious. Every few months, when someone gets a new phone or accidentally logs out of his account, we need to meet in person to compare fingerprints.

Of course, even if you leave out the E2E encryption, it's still about as secure as Telegram's default settings. And with E2E encryption, it should be about as secure as Signal is (I'm not a cryptographer, though, so take this with a grain of salt). However, if the user is not experienced with key verification, I'd always recommend Signal over Riot.

> But if you want to use E2E encryption, the key exchange is really tedious.

Agree. But that part is still alpha in current release, so I hope they will improve over time.

+10 for telegram

Telegram server is not open source right?

Unfortunately, currently not all about Telegram is open-source: https://telegram.org/faq#q-why-not-open-source-everything