You could build an ontology in RDF [1] or OWL [2] and link the grammars together.
FOAF [3] was a means of describing your friends and social connections, and it allowed rich annotation.
No tools were ever built around this because by the time it started taking off, Facebook and MySpace were already a thing.
The great thing about these technologies is that the graph is public and you could write tools to ingest, export, and exchange the information. We never got to that, though.
The technology was centered around graphs and triple stores, allowing you to apply subject-predicate labels to anything. The URI became a central node, document, and ontology identification scheme.
There were languages like SPARQL for querying rich triplestore databases, but they never really took off. Again, Google and Facebook were already huge by the time these started to mature.
If you think the promise of blockchain is exciting, you should take a look at what the Semantic Web could have been...
[1] https://www.w3.org/TR/rdf-primer/
Which is probably why it failed. Richer means more complex, means higher effort, means harder to justify the immediate effort.
It also means more difficult to figure out which sources are trustworthy because it all gets mangled together. The success of google is not that they are good at indexing, but good at ranking. People are constantly trying to game the search engine system. The naive model of querying a semantic web builtup doesn't mesh well with a certain percent of participants being malicious.
And that's excluding the whole problem that scaling searching large complex graph databases is basically an unsolved problem
My impression is that hashtags grew from people on Twitter. A group of people started putting # in front of keywords in their tweets, to mark them as central to the topic of the tweet, so that others could search for those keywords with the # in front and find only tweets that were specifically about that thing rather than happening to use the same word while talking about something else. Twitter staff noticed and the hashtag was turned into a feature. Other sites then adopted the hashtag feature too.
Twitter was the first to introduce inline tags with the hashtag, and I believe you're correct that it was user-driven behavior.