@ricardojmendez A common theme of the most successful forums I've seen is that first and foremost they addressed some extant group or community's real needs. Usenet, BBSes, mailing lists, Slashdot, Reddit, Facebook, Twitter. Probably Instagram and TikTok.
And that extant group had at least some level of aspirational appeal that others would be drawn to.
Dates refer to formative periods, not the full reign of the mode
Usenet (1970s -- early 1990s) was mostly technically-inclined college students (largely graduate), faculty, and staff, as well as some tech/military sector companies and a few government departments.
BBSes and dial-up services (late 1980s -- mid 1990s) saw strong early growth amongst military families dealing with overseas stationing. I'd always sensed a somewhat stronger theme of this from that community, but it wasn't pointed out to me that these services explicitly marketed to military families until decades later. Cheap fast comms of a widly-distributed and large population.
The WELL (1980s) was a BBS catering specifically to personal computing enthusiasts around the existing Whole Earth community.
Mailing lists (1980s -- early 1990s) initially addressed programmer and technical-user needs to share development and peer-based technical support, as well as of distributed academic groups. (Email was largely limited to the same groups as Usenet and drew heavily from the same cohort.)
Slashdot (late 1990s) was free-software geeks looking for a news / discussion source which wasn't beholden to Microsoft and its desktop monopoly and anti-Linux FUD, as well as the burgeoning set of dot-com / Web1.0 companies and technologies.
Digg and later Reddit (early-to-mid 2000s) were something of refugee communities from early Slashdot, and both largely saw en-mass defections to themselves. Arguably that crown's now passed to HN, which similarly revolves around tech and startups much as Slashdot once had.
Facebook (2004--8) was Literally Harvard, the Literally Ivies, then Literally Selective Universities, then Literally College Students. It based growth out of a young (long-term habit-forming) and appealing (both to other users and advertisers --- educated & high income potential) group.
Twitter (2009--2014ish) seems to have largely appealed to media in both researching and publicising news stories, as well as interacting amongst themselves.
Instagram and TikTok (late 2010s / early 2020s) seem to appeal strongly to the celebrity community --- pop music and cinema largely. (It's still early, I'm out of touch, this may well be wrong.)
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