People jumping to signal likely are looking for privacy. The only app that guarantees that right now, with minimal configuration, is Signal.

Jumping from Facebook or WhatsApp to Telegram doesn't make any sense from a privacy perspective. Well Telegram supports secret chats, It unfortunately defaults to non-secret chats, which are just to secure as any old Facebook post. In fact, if we're comparing WhatsApp to Telegram, from a privacy perspective, you would be giving up more data by switching to Telegram because WhatsApp is completely end to end encrypted by default.