I just returned from Web 2.0 Expo in San Francisco. I heard several talks on social media strategies. One excellent presentation from an executive at LinkedIn discussed suggestions for creating a social network.
The most successful social networks start with a simple concept--one or two activities in which members can participate. For example, Flickr--posting or viewing pictures, UTube--posting or viewing videos, Facebook--posting pictures, LinkedIn--connecting to other people etc.
Along those lines, the speaker recommended that any social entrepreneur begin by considering the following:
1) Audience: Who are and why would they want to participate in the community?
2) Theme: What is the focus of the site?
3) Objects: By objects I mean what members should act on... video, photos, stories etc.
4) Activities: What are the actions that members take with each of the above objects? Examples can include: sharing, posting, viewing, forwarding etc.
By beginning with a clear audience and simple activities, the site will have the best chance of building an initial following of devotees.