I've been looking at using one of the social networking services to extend the reach of a growing community of ~200 people. We don't want an email group or listserv experience, and the group has a website where they post announcements and other "broadcast" material. It seems like a social networking service might be a good extension, particularly in light of the desire of the community to be able to reach out to the more distant members.
I've started thinking that LinkedIn for Groups is the thing to use. A number of the community members already have profiles on LinkedIn, and LinkedIn seems a reasonable service for business-focused networks. The primary thing I see LinkedIn groups providing is an affinity group for this community. They don't all know one another, but the fact that they are all part of the same community has some value, and they would like to be able to take advantage of it.
There are several other options out there, as always. They could move to something more centered on their own website, but that removes the opportunity to reach out based on the second and third level connections people represent in LinkedIn. They could go with Facebook Groups, where there is more interactivity built in, but less "social networking" it seems to me.
I suppose a core question for this community is what they think they want. That hasn't been completely clear to me.