The significance of this feature is that we can leverage even more decisions about who’s worth listening to. For example, compose a bunch of lists tracking a breaking news story (like the Fort Hood Shootings last week), and track all of the lists simultaneously.
We’ve also started tracking changes in the social graph, so you can see who’s getting added to these lists. In the case of breaking news, you could use this feature to see who are the new sources being discovered. Twitter doesn’t provide a streaming API for social graph changes, so we resort to polling for updates hourly.
The addition of lists was a technical hurdle, since we’d built in the assumption that the nodes in our social graph are users with twitter_ids stored as integers. We’ve expanded our definition to be more generic, and thus can now incorporate twitter lists, and potentially other kinds of attention focal points.