Forums vs. Social Networks?

I'm debating the value of forum / group / threaded discussion software vs. social network software for a particular situation.

It aims at a very broad audience that includes everyone from early adopters to technology laggards.

We generally expect users to break into:

• 5% - Heavy Users - spend quite a bit of time and heavily participate
• 15% - Light Users - spend a little time, participate a little
• 80% - Fly-Bys - spend very little time, read bits and pieces

It is likely that some of the Heavy Users will be technology laggards, but they will want to participate because of the content.

Participation will eventually mean a lot of different things, but initially it will primarily be sharing ideas. A user can do this through posting to a threaded discussion quite easily. They could also do it as comments on a blog.

So, I'm trying to figure out what's going to be the right software / service to adopt, but I'm also trying to think about the differences in:

Forum / Threaded Discussion / Group

Examples are Google Groups and Yahoo Groups. Typically they center around threaded discussions. Generally are easy to get into. Options are simple.

Social Network

Examples are Ning and KickApps. They center around individuals. They form a network of people via interactions, groups, etc. Being a network, they generally are a little harder for people to understand. However, they typically try to give a better sense of the person and make interactions more social.

The distinctions here are horribly gray and most of the services end up with fairly similar offerings. A lot of what it comes down to is the primary view you show users - classic threaded discussion view - or a personal home page with all that is happening in the network.

Some thoughts I have about why we might adopt threaded discussions / forum software vs. adopting a social network solution.

• Longer-term we want to have more of a social network where people will become more social, interact in a myriad of different ways, create groups within the site, and generally will take it into classic social network realms.
• Short-term we want this to be really simple to get into. I'm especially concerned about the user who would want to be a heavy user - contributing lots of ideas - but who has never used a social network. Possibly they've never used threaded discussions either.
  

What do you think I should be considering here?

Clearly, there's a lot more to it than just the software. It's how we use the software. It's providing the necessary direction and hand holding. It's having people ready to help/guide/mentor users.