Although the name implies that Facebook Docs is something similar to Google Docs, it really is nothing of the sort, at least yet.

At the f8 conference yesterday, Facebook took the wraps off of a way to share documents, spreadsheets and presentation files online with your friends.   The service is called Docs.com and powered by Microsoft's Fuse Labs and is currently in Beta.  The permissions for the Docs are granted and controlled entirely within Facebook.

Otherwise, the Docs are similar to Microsoft's Online Office Live product, with the ability to build Powerpoint, Excel and Word documents online.

If you take a look at the service however, it is not much more than another Facebook app that shares files.  You can edit files online but there is no collaboration like Google has had for months and recently updated to real-time collaboration.

From Docs.com help pages

In fact, this product matches up more with Apple's iWork.com website, which also offers Facebook built-in sharing but permissions are given out with Apple IDs.

You can share your Google Docs via a URL link in Facebook, but cannot control who has permissions to edit it in Facebook.  To edit, they'll need to log in with a Google account.