From W3C Wiki

Popular social platforms use various microsyntaxes to for example mention someone else. Possibly the most popular @ mention used by Twitter, Discourse, Trello and many other systems. Other systems use other symbols, Google+ uses +, Tent ^ etc.

PROPOSAL: Systems generating activities MUST set all the relevant data attributes as specified in ActivityStreams 2.0 Vocabulary. This means system used for authoring AS 2.0 objects, MUST NOT rely on any other system parsing some additional data out of textual contents used as property values in JSON serialization. For example one could use any of @elf , +elf, ^elf as long as it sets as:to or as:cc value to something in lines "to": { "@id": "", "name": "elf Pavlik" }

based on email thread

See Also

Previous research and work on microsyntaxes: