In addition, while I’m not an expert on local use of these things, Steven Clift with http://forums.e-democracy.org/ does something similar as well.
My take on this is that local efforts really need to be developed locally. The technology is not nearly as important as developing something that is appropriate for the citizens that you’re hoping to engage.
Just so you know, Facebook does allow you to limit group membership, both by invitation/requiring people to request access and by network. Network restriction may give just the ability you’re searching for.
If you’re not familiar, you should also look into CityCamp, the Open311 projects, and Localocracy (who is part of this community as well: @conor).