It runs wherever there is used generated content. It is user-controlled, even in corp-controlled platforms.
Wherever users can write text, they can [[annotate]] as per an established convention.
[[wikilinks]] are ubiquituous [[tags]]. Browser extensions can easily auto-link them to whenever the user wants to; for example, to a hub repository of them (https://flancia.org/go/agora-protocol is my current proposal, but that's just a detail; the most useful convention should win).
Images (like those you can attach in, say, [[twitter]] or [[mastodon]]) can encode information; think of attaching QR codes to every tweet using a custom client). Or just screenshots, optionally post-processed (OCR'd) or making use of [[stenographic techniques]].