Telegram: 200M Monthly Active Users
Despite Telegram's shortcomings in not using well supported crypto and not having default end to end encryption on all conversations. Their clients for both iOS and mac are amazing in that they are native.And unlike WhatsApp you don't need to carry your phone with you all the time just to send a message to someone on desktop.Their bot API is also really rich. I am thinking of rewriting some of the Alfred workflows I wrote in Go as Telegram bots. For example I always wanted my Web Searches workflow (https://github.com/nikitavoloboev/alfred-web-searches) be present on my phone in some way. It seems I can actually do just that and reuse the Go code I wrote already.