https://www.w3.org/TR/activitystreams-core/#jsonld

says

“The serialized #JSON form of an Activity Streams 2.0 document must be consistent with what would be produced by the standard #JSON-LD 1.0 Processing Algorithms and API [JSON-LD-API] Compaction Algorithm using, at least, the normative JSON-LD @context definition provided here.”

So that means that Activity Streams should have a good extensionality story, and so adding tags with semantic content should not be too difficult.

Re tags. There was a company called Faviki that did a very good job of linking #tags to Wikipedia, via DBpedia, perhaps. This allowed people to come to agreements over tags in a communal way, and it allowed one to search for similar tags or implied tags. (also it could help with translation between tags)

I wrote about Faviki somewhere on blogs.sun.com, but I did not find it. Here is one thing I wrote on the subject:

http://blogs.sun.com/bblfish/entry/search_tagging_and_wikis