Types of social network

One thing I'm thinking about at the moment is how I'll structure the first version of the social network I'm building. Social networks come in a lot of flavours, from the messy, real-world face to face social networks that we all live in, to online discussion forums, where people meet to chat about things, to the more formally structured Social Networking Sites, where people make explicit links between each other. I'm most interested, from a research perspective, in the explicit social networking site in the context of a virtual learning environment; but even then, there are a lot of choices in how to develop the site:

1. Is it a "pure" social network, where the interaction and linking between people is the primary purpose - Facebook feels a bit this way (though it has a lot of other features, mostly people just post statuses and photos, and their friends like and comment on them), and this seemed to be what Orkut was mainly about, at least in the early days (I'm not Brazilian, so I haven't been to Orkut in years). Or is it some other site with a social network built around it - like Flickr, or Tumblr, which are mostly about photo and video sharing, but have the ability to connect to other users in order to keep up with what they are posting?
   My initial plan is that I'll be developing a resource-centric social network. Probably the major reason for this is that I want my participants to have a reason to use the system. They are students at a major University, all in the same course, and so they already have a strong social network among themselves; giving them a pure social network just replicates what they already have in Facebook. The other main reason is that I'm aiming for this to be a social network for learning - making it resource-centric means that they will be sharing learning resources that they have created or discovered.
2. What is the structure of the network? Here I'm talking graph theory - not this kind of graph:
   
   but this kind:
   
   Imagine each of those dots is a person, and the lines are connections between them. Does a link need to be agreed upon by both parties (as in Facebook), or can it be initiated by only one party (as on Twitter) - ie. will the graph be directed or undirected? Do the links have additional properties (as in Google+'s Circle feature; or Facebook's ability to make certain links as being family or spouse)? How many links do we expect a typical user to have, and is there a limit (Facebook allows a maximum of 5000 friends; Twitter and Google+ are unlimited) - ie. what are the typical and maximum outdegrees of the network?
3. What does a connection mean? Does making a connection to another person in the network imply they are a "friend" or a "study partner", or is it more an "I'm interested in what you're posting"? How I name and manage the linking function will suggest to users what is meant by it, and hence affect the way the network is used - people will react differently if they're "following" someone's posts, as opposed to if they're declaring them a "bff", even if the functionality exposed it identical.
4. What kinds things can be posted into the network? Is it plain text, (à la Twitter), or is it a multimedia extravaganza (à la Tumblr)? Does each post lead to a discussion (Facebook), or can it only be liked or re-shared (Twitter, Tumblr) and replies need to be posted in the replier's own posting stream?
5. How are posts rated by other members - are only positives allowed (Facebook "like"s, Twitter "favourites"), or are negative ratings also allowed (Youtube's like/dislike, Slashdot's moderation tools)? What do the ratings mean? Do they have additional properties (Slashdot's "Funny", "Informative", etc. ratings)?
6. What are the posts attached to - are they attached to a person (Facebook's statuses and walls), or are they attached to some other things (eg. Facebook's groups, Flickr's Sets, Tumblr's blogs)?

There are many other more subtle choices that affect how the system is used. So, where to from here?

My objective with the system I'm building and the research I'm doing is to build a social network that students willingly use for learning. I'm lucky in that I have a large cohort of very keen students who, if presented with a sufficiently useful tool, will use it. I'm building this into an existing course in an existing LMS, so it needs to follow the conventions and requirements of those. At this stage, my thoughts are that the most useful thing for students is a resource sharing network - they're all out there finding useful web sites, creating summaries of learning material, and so on, and being able to share these could be very useful. The most obvious way to handle the connections is to allow students to follow other students, so that they can build up a feed of interesting new resources. This means building a directed graph, and that the meaning of a link is of the "interested in your output" kind. It means allowing student to post anything into the network (Tumblr style), since it's impossible to predict what the most useful kinds of resource will be. It means building a rating system to allow constructive criticism and collaborative moderation. And it means using the curriculum as the crux of interaction, in the form of Learning Objectives.

Of course, all of this could change, depending on how the students take to it, and what their actual needs turn out to be. It's very unlikely I'll stumble across the Right Solution first time around - it's much more likely that I'll have to try a number of things, until I find something that the students take to. This kind of research is very context sensitive, and (as shown above) there is a large space of possible social networks, some of which would work very well in other contexts, and a few of which will work in my context; so I'll need to wander around, looking for that local maximum of affordance and functionality.