Based on real problems I encounter through my work: - A simple commenting system - A simple up/downvoting system - A simple forum - A simple RSS-to-newsletter tool There are solutions in that field, but they are usually way too complicated, or can't seamlessly integrate with an existing website.
Feedback is super important. That's how I scaled an app from 1k to 3k active users in a month. We had an internal "chat" hacked onto Firebase, and it turn the app from a recipe book to a community. People complained a lot about the app via this chat. We fixed the problems quickly and they'd recruit people from their other communities. It was also an exhaust pipe for complaints; we'd get few bad reviews on Google Play because people were heard.
SMF and PHPBB cost too much and were designed for an era before 80% of people were on mobile. Reddit and Discourse creates groups of people, not communities.