Secure, end-to-end, multi-device encryption isn't easy. Plenty of people try and fail to build secure messengers based on top of PGP and Signal's protocol.

I don't use the Telegram web app, but their native apps work excellently. The insertion of ads has been a major disappointment but the chat UX itself is still great, even on native Linux.

But Telegram end-to-end encryption is optional as far as I know.

It is, and it's a good reason to avoid the platform, but they do support E2EE _and_ have a good, native desktop application.

> have a good, native desktop application.

Indeed. I'm not actively using Telegram, but I tried the desktop application (made with Qt if I remember well), and it's way ahead of what Whatsapp offers. Not to mention it's fast and relatively light.

It's made with Qt indeed, the source code is here: https://github.com/telegramdesktop/tdesktop

Facebook could just take the app, change the colour's to make it green, and replace the messaging protocol with their WhatsApp library, and they'd get an actually usable chat client practically for free.