Elgg is a social networking platform in a box - think of it like your own private, white label Facebook. It has many of the same concepts including friends, groups and an activity stream. There are a handful of widgets available for it which enable features such as:

• Blogs: write posts that are shared amongst specific groups for friends
• Twitter: integrate your Twitter account so Elgg bi-directionally aggregates your Tweets and flows Elgg posts out via your Twitter stream.
• Wire posts: post sitewide notifications that are available regardless of group membership.
• Flexible notifications: customize your email notifications so you receive update messages based on activity amongst certain groups or friends.

If you are looking to host an in-house Facebook-like system Elgg is a great option and the JumpBox for Elgg provides the easiest way to get started with it. Check out the video below for a quick 5-min orientation to how it works.