The "Follow" Activity type lets us model unidirectional connections -- Evan following James means that Evan gets updates of James's activities, but not necessarily vice versa.

Many social networks (Facebook, LinkedIn) have bi-directional connections - one user "requests" friendship from another user, who then "accepts" or "rejects" it. Afterwards, both users get updates of each other's activities.

It would be good to be able to model this schema, since it's pretty common on social networks.