In Activity Streams, verbs are their own objects, and the variety of actions that can be represented is limited only by the standard itself. Providers can also use verbs outside the standard, taking the chance that they'll eventually be incorporated, or that a downstream client could parse them anyway. Here's a list of the verbs incorporated in the Activity Streams standard so far:
The “thin portfolio” concept (borrowing from the prior “personal information aggregation and distribution service” concept) represents the idea that you don’t need that portfolio information in one server; but that it is very helpful to have one place where one can access all “your” information, and set permissions for others to view it. This concept is only beginning to be implemented.
By filtering your results by popularity, you'll be able to pare down a bunch of results that are presumably all relevant into the top sites on your topic, or some surprising ones you might not have heard of.