Because, if you are ever lucky enough to be one of Facebook's 8,348 employees, you will be on Facebook all day long. Not with your friends, but with your co-workers.
At Facebook, employees use Facebook for everything: they use it instead of email, as their group chat room, to collaborate on documents, to IM, to share news.
Sources close to the company tell us a few other companies are in a pilot mode, testing a version of Facebook at their companies that mimics how Facebook employees uses it internally.
For instance, if someone at Facebook notices something on campus that needs to be fixed, they post a photo of the broken thing to the facilities management group.
In fact, through such photos, the maintenance crew noticed that employees were always cutting across a certain section of grass. So they put a brick path in. Which caused a Wizard of Oz prankster showdown.