Circles, Smart Lists are they enough? the case for Topic MessageBoxes

Google+ introduced the idea of sharing your social updates with a dynamically created subset of your friends (e.g your "circles" of friends). This was a kind of evolution of previous Facebook model where the user was allowed to share just with statically predefined groups (e.g, just my direct friends, just my friends of friends, or everyone). Facebook answered to "circle" by presenting "smart lists" which are essentially doing .. exactly the same thing.  Well, sometime I fell like both circles and smart lists are limiting to my ability to share. Let me explain why...

One of my hobby are the "Acquaria" so let's suppose that I want to share an update about this topic. What I need to do? 

• Is this private? so just for my direct friends? probably not because many of my friends are not sharing this hobby and I don't want to bother them;
• Is this for my friends of friends? hmm, I would say no because of the same reason;
• Is this public? certainly it is not, it would increase the noise received by many users and just few of them can see any value in my message;
• Should I create a circle/smartlist for this topic? Hmm, maybe..  but hey wait! are you saying that I need to create a circle for each topic associated with my status updates. Do you know that creating circles is expensive and boring? I cannot pick all the users i want for each single message -- this is not functional;

What I simply want is to share a message and say that this message is about this topic. If you are interested in the topic you should get the message, if you are not interested in the topic you should not get the message. As simple as this. Plus, you maybe interested in a topic even if you are not (already) my friend  or a friend of my friends. We don't know each other, but you may still see a value in my message and have a good experience.

You are probably following me by now. Suppose that I put my message in a messagebox and there is a label associated to this messagebox describing the topic of the contained messages. I don't have the explicit need of saying who are the recipients of my message I just indicate the messagebox. If you are interested in that topic you just open the messagebox because you already expressed your interest. And I have the freedom of choosing the appropriate messagebox as I like.

There are several interesting problems for this new model based on topic messagesboxes.

First, how do you rank the messages for each messagebox? Well, I envision a combination of personalization techniques and user ranking. If the messages is coming from my friends than this is a strong signal of preference, if a message is coming from a person that I don't know but this person has been already considered authoritative for that topic that this is another strong signal, if a message is coming from a person that I don't know but is authoritative person than this is another strong signal. All those signals can be combined together using some regression model such as Gradient Boosting and can be evaluated according to the interactions generated for each message. Also, I need to define a user ranking based on his previous scores. All of those are interesting ML problems. (yes, Ian this would be your dream i know).

Second, how can I generate the topics associated to the messagesboxes? There are multiple options. I certainly want to have a set of predefined topics (maybe suggested via an autosuggest feature). In addition, I want to give to the user the ability of defining new topics. In any case, I need to rank those topics according to the user past preferences.

Third,  I want to have the freedom to subscribe to specific messageboxes and get updates as soon as new messages arrive. This would be a powerful way to explore the system and get value from the messages even from unknown people. If you think about it, this is also an opportunity of creating new friends.

I believe that you are following me by now. 

So let's repeat the above experiment and let me introduce Maya she has a passion for {romantic movies} and she wants to get as much suggestions as she can about these movies. How can you help her? Circle, Smart lists or Topics?