It's funny that when I was originally thinking about Activity Streams, around 2008 or so, it seemed very clear to me that the application for "reading your friends" and the application(s) for creating content would be two different things, with the former being a largely-read-only (aside from likes/comments, perhaps) generic thing that supported many object types.
Of course, that's because I was still "RSS/Atom-brained" at that time, thinking of enriching feeds.