Thanks to Techcrunch for the following analysis of the various offerings out there. The main differentiators are:
cost – initial, on-going, development (their team vs. your team), hosting (their servers vs. yours);
hosting – their servers or yours (if you care or worry about where your sensitive data sleeps at night, you may need to consider your own hosting.);
features – group discussions, rich media, integration with other social networks, membership management, on-going applet/widget development, scalability – e.g. maximum number of members supported, customization options;
Here they are:
Lots of information to plow through but a good starting place to evaluate what options you have available.