There is a competitor to Ning: CrowdVine. However, it seems to be marketing primarily to conference and expo organizers at the moment.
Also, this old PC in the den has gotten slow again, even with AVG installed. Now I’m considering switching it back to bloody Ubuntu (again).
EDIT: more competition:
- Elgg Spaces
- TheVillage
- Insurance Networking (wtf?)
- GoingOn
- PeopleAggregator
- OneDomus
- Flux
- six groups
- Elgg.ogli.net
- stage4.us
- WackWall
- A longer and more complete list of similarly-competing services.