Skybrud.Social is a framework in .NET that integrates with a number of social services.
This package - Skybrud.Social for Umbraco 7 - will use Skybrud.Social for bringing stuff like OAuth authentication to Umbraco 7. The package focuses on the Umbraco backoffice by adding a set of data types. As of now there are data types to handle the OAuth authentication for the APIs of Facebook, Twitter, Google and Instagram.
You can then use Skybrud.Social for interacting with the supported APIs.
Even though Skybrud.Social for Umbraco 7 is a package made for Umbraco, there is a good chance that you only need Skybrud.Social.
For instance authenticating with the Facebook API will give you a user context so you can make calls to the API on behalf of your user and access private data your user has access to.
If you just need to access public data, Facebook (and the other APIs) have what is called an app context, and you don't need the Umbraco package. You can consult the Skybrud.Social documentation for further information on this ;)
Similarly if you want something that lets your members (visitors) log in using Facebook, Twitter or similar, this is not something that is supported by Skybrud.Social for Umbraco 7, but you can implement this on your own using Skybrud.Social.
The package is available on NuGet as well. This is the recommended approach to installing the package.
You can mfind documentation for Skybrud.Social at social.skybrud.dk
This package is created for Umbraco 7, and therefore doesn't support Umbraco 6.
I do however have an old equivelant for Umbraco, 6, which you can find at: http://our.umbraco.org/projects/website-utilities/skybrudsocial-for-umbraco