It was pretty cool; you'd think it would have taken off like a rocket.
Is someone still doing this in some shape?
Outside of advertising, we don't have good business models for discussion/social software with large user bases.
Some vertical sites like HN and forum conglomerations like Reddit have survived with relatively stable audiences. Post-Twitter fragmentation continues. Social media censorship and surveillance have driven some discussion into smaller groups of known participants, including WhatsApp, Telegram and Discord. Genius.com invested millions in music annotation and tried expanding to news, but eventually folded, https://en.wikipedia.org/wiki/Genius_(company)
Subscription and donation-funded newsletters and videos are arguably a form of annotation to "comment on anything, anywhere", but are still subject to the vagaries of algorithmic promotion and discovery. TikTok videos could be considered a form of permissionless annotation, fusing text/audio/video into unique styles and viewpoints, building an audience for commentators.
(if anyone were to make another attempt, please allow annotating annotations? eg https://www.sas.upenn.edu/~jtreat/rs/002/Judaism/talmud.html )
[but beware of the self-annotation, eg https://news.ycombinator.com/item?id=41739259 ]