On his blog Christoph Rumpel explains how you, using BotMan, can create a bot where users can subscribe to content updates.

Email has been a great channel for staying in touch with your audience for years. But times are changing and messengers are taking over communication. Let's see how I used Facebook Messenger to build a little newsletter bot for my upcoming book "Build Chatbots with PHP".

https://christoph-rumpel.com/2018/02/build-a-newsletter-chatbot-in-php-part-1

Pretty cool! I might add a bot to this very blog soon.