Honestly, I think a lot of different services and systems have hurt forums popularity wise, though it happened in stages rather than all at once. Facebook Grou

Honestly, I think a lot of different services and systems have hurt forums popularity wise, though it happened in stages rather than all at once. Facebook Groups did damage to communities aimed at older and less savvy audiences. Stack Overflow likely replaced quite a few tech communities (as did its spinoffs in that network). Reddit replaced a lot of forums for niche topics and hobbies. And Discord seems to have taken over for a lot of 'young person focused' topics like games, film, tv shows, web series, etc. Discord has basically annihilated about two of those services as well as many smaller communities too. Stack Overflow is losing out to Discord servers (and possibly Slack instances) aimed at the users of a specific framework or language, and many of Reddit's communities seem to be switching over to there too.