I think that trying to replicate such a system in XF based on "Following" people, as it's currently working, is less effective than it could be.
I suggest that instead the Admin have the ability to specify a "Like" threshold. Once a member (A) has liked that many items of content (posts, blog entries, gallery images, status updates etc) from a given member (B), member B's content automatically starts showing up in member A's activity stream. If member A chooses they can simply remove member B from their following list.
That way member A's liking content (which we've seen is highly used, FAR more than "Following" people) shapes what content they are shown in their personalized view of things that have happened in the community(maybe in a sidebar? ).
Facebook is based on specific, conscious decisions to establish a link. But people in a forum based system aren't focused on that sort of thing. They are reading content about a topic that they are researching and or interested in. But by using an the popular and FUN "Like" system, there's a chance to GET them engaged on a more personal level by drawing their attention more firmly to people that they LIKE.