Seeing that WhatsApp is not prohibited in Russia is a big concern of itself. This country has a law that requires you to pass ‘master key’ (or a backdoor, if you read between the lines) to special agency, or be banned by all ISPs. When my friend researched the topic, some interesting guys told him few years ago that whatsapp is a no-brainer for them to read, but tg+secret is opaque. Subsequent events shown that it was probably true.

All this is full of false negatives and positives, but concerns too much to ignore, if you’re after secrecy that puts your welfare at stake. If you just hide your pics from a corporate big brother, I admit that whatsapp and regular tg chats may have different weights on scales.

And you just trust Facebook to somehow really encrypt everything in this black-box? I think this is naive as Facebook has always lied to us and proved it isn't a trustworthy entity.

WA used to steal your contacts by default to improve Facebook's data set and probably still does.

The Telegram guy however left his country because he didn't want to cooperate with such entities (like the Kremlin which for me is kind of another organization that seeks to harm your privacy among other goals and is therefor comparable to FB and other spy-companies).

I thought Telegram was sending messages encrypted (though not E2E) per default?

It isn't by default. You have to enable it yourself.

This is wrong. All communication is encrypted in transit, even for non-secret chats.

Is it also E2E encrypted though? If not, wouldn't that mean that Telegram servers could snoop in on the non-E2E conversations?

You're talking about "secret chats", the Telegram way of doing E2E, but it doesn't mean that all other communications are sent or stored plain text. And there is no setting in the app to enable encryption (I just checked), because there is no way to disable it. "Not E2E" != "not encrypted"