Slack’s API is obviously very popular. Facebook’s Messenger platform and WhatsApp have huge distribution but are shadowed by Asian rivals like WeChat and Line. Telegram’s Bot API shouldn’t be overlooked. Google may yet offer its own chatbot messaging platform (superseding Hangouts?).
Well organized development teams will grow their service commensurate with their customers’ needs, moving at their speed, and without being held back by opaque app store submission processes. When the new version is ready for release, the code is pushed and instantly every user gets the latest version. No updates, no installs, no delay.