Ning

Advantages
You can create your own social networking site.
You can choose your own web address (it won’t have a bunch on nonsense letters or numbers unless you want it to).
You can fully customize your network.
You can list upcoming events on your network.
You can make groups within your network. For instance, a library network can have groups for children, teens, and adults (or more specialized groups within those categories).
Ning networks provide opportunities for discussion forums, widgets, photo sharing/slideshows, activity streams, videos, and Facebook integration.

Limitations
Ning can run ads on your network pages unless you pay them not to.
Ning networks have 10GB of storage and 100GB of bandwidth unless you pay for a premium service upgrade.
If you create a highly specific open SNS on Ning, it may be less likely that people will find it.

I believe that Ning would be very useful in a work environment. Coworkers can have discussions and stay in contact with each other easily using a Ning network. People from different departments and even different office branches can stay connected. It’s also cost efficient because a Ning network is free (unless your company/organization chooses one of the several upgrades). Even though it’s an online community, you can choose who to let in. You can have an open network and block unwanted users. Or you can run a private group on a private network and have members join by invitation only.