The genre as a public-facing thing has roots in pre-internet trivia almanacs — The Book of Lists (1977), Schott's Original Miscellany (2002), and the broader "useless information" book genre that peaked in the late 1990s. The web equivalents started appearing in the early 2000s.
Mental Floss launched as a print magazine in 2001 (founded by college students Will Pearson and Mangesh Hattikudur), moved primarily online by 2010, and became the canonical editorial fun-fact publication. Atlas Obscura launched in 2009 as a niche curio blog and graduated into a full editorial publication by 2015.
The 2010-2018 era was the genre's first proper expansion. Today I Found Out launched in 2010 with a daily long-form fact format. FactRetriever launched as a verified-fact aggregator with explicit source-tagging. WTF Fun Facts went social-media-first and brought the format to Instagram and TikTok.
The 2020-2026 era has been the consolidation. Mental Floss survived two ownership changes and remains active. Atlas Obscura grew steadily. WTF Fun Facts emerged as the social-media-native variant (Twitter/Instagram-first, with a website mirror). FactRetriever launched as the most-explicitly-verified fact engine, marking each fact's source and verification status.