We use Yammer at our two-person company. It's not like Lotus Notes or other groupware I've used in the past, but there are similarities to Facebook and document-sharing tools.

We find utility in the ability to have asynchronous discussions among a restricted group of people and share links (like Facebook) and also to share documents and other files. A typical use case is someone posts a file, a link, or groups of files or links, and then we discuss them.

We could use email for the asynchronous discussions, or Dropbox or Google Docs for document sharing (and we do, sometimes, for some scenarios). But having all of these resources in a single, searchable, discussable location is very useful for us.