One of those companies is Zerista, which is a tool that lets groups, clubs, teams and organizations build their own mobile-optimized applications. Think of it as a private Facebook/Foursquare/Twitter mash-up.
What we like about Zerista is that it can integrate with your existing social networks, while also creating a web-based (and mobile optimized) network for communicating news, sharing messages, doing mobile checkins and even collecting payments for tickets or dues.
You create the application and provide details about who your members are, any custom maps you want to include (that can replace generic Google Maps) and any activity calendars or events. You can also select what type of privacy and sharing options you want -- so you can designate if communication shared in the group is also shared outside to other networks or to the public at large.
While some might not see the point in building a custom social app, we can already think of several situations that would really benefit from such a system. For instance, I was in a sorority in college and trying to track down meetings, locations for philanthropy events -- not to mention collecting dues -- was something that even in the early Facebook era wasn't well optimized.