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Yes, and enabling E2E "secret chats" on Telegram actually hurts user experience, so most non-tech-savvy users avoid it. Additionally, your address book is continuously synced to Telegram servers.
> there’s little preventing Facebook from pushing an update to your phone which backs up all your chats to their cloud
Just to add to your point- WhatsApp currently backs up your chat history to Google Drive or iCloud unencrypted. It requires opt-in but they nudge you frequently.