Think about it: Lemmy provides you with a ready-made frontend and backend — all you have to do is host your own instance of it. The following could all have been implemented as Lemmy instances, had it existed at the time:
- Reddit (www.reddit.com)
- Quora (www.quora.com)
- Stack Exchange (stackexchange.com)
- 4chan (www.4chan.org)
- Disqus (disqus.com)
- Hacker News (news.ycombinator.com)
- Something Awful (forums.somethingawful.com)
- GameFAQs (www.gamefaqs.com)
- Bodybuilding.com Forum (forum.bodybuilding.com)
- Gaia Online (www.gaiaonline.com)
- IGN Boards (www.ign.com/boards)
- NeoGAF (www.neogaf.com)
- MacRumors Forums (forums.macrumors.com)
- Digital Spy Forums (forums.digitalspy.com)
- WebmasterWorld (www.webmasterworld.com)
- AVForums (www.avforums.com)
- DeviantArt Forum (forum.deviantart.com)
- XDA Developers Forum (forum.xda-developers.com)
- WordPress Support Forums (wordpress.org/support)
- The Student Room (www.thestudentroom.co.uk)
Of course, these all have very different rules and frontends, but those can still be changed.
In addition, members of other instances can visit these forums without having to create new accounts, thanks to everything being federated.
Isn’t that cool?
What makes you think everyone was doing it from the ground up? vBulletin, and phpBB were both released in 2000, and Simple Machines was released in 2001. If a forum was custom, it was for a reason.
Also, more recently Discourse which has become pretty popular.
Just make sure you install Categories Hierarchy before you install Attachments Mod, or you’re gonna have a bad time.