More rereading of Alexander: 129 Common Areas at the Heart
No social group- whether a family, a work group, or a school group- can survive without constant informal contact among its members.
Create a single common area for every social group. Locate it at the center of gravity of all the spaces the group occupies, and in such a way that the paths which go in and out of the building lie tangent to it.
This is why groups is so important to social networks. I question Facebook’s radical de-emphasis on groups in their redesign. Gossip tells me they are trying to move away from groups, since it is on the old architecture. I doubt they will ever get people to give up their groups. LinkedIn is investing heavily in group functionality, with regular updates and additions supporting necessary social activity such as discussion. Yahoo groups, great grandfather of them all, continues unchallenged growing steadily. You need common areas at the heart of your system, because common areas are at the heart of your users.