One of the more promising parts of the coming upgrade of Microsoft Office is the online version — but the responses I’ve seen to sneak peeks of the web-based Office apps have been pretty mixed. Beyond the Office brand name (which is certainly formidable), what’s going to make these apps any better than what companies like Google already offer?

Well, Facebook chief executive Mark Zuckerberg just shared a feature that could give Microsoft a big advantage over competitors. Microsoft and Facebook launched a site called this morning, which is all about sharing your documents using Facebook in the cloud. He didn’t offer too many details, but said the document site will work closely with the social network. If you want to share a document on Facebook, you can do so using the connections you’ve already built on the social network.

Here are some quotes from the Microsoft post introducing the site (which is formatted, rather annoyingly, as an online Office doc, making it much harder to copy-and-paste):

Built on Microsoft Office 2010, the Docs app enables Facebook users for the first time to create and share Microsoft Office documents directly with their Facebook friends, using the Office tools they already know. …

This exploration exposes yet another facet of Ray Ozzie’s broad and long-held vision of social productivity. Although quite simple (and fun!), and its Facebook app bring together the best of “software” with the best of “services.” Documents can be created directly within the app, or can be uploaded from your PC. After starting work on a document alone, you might then choose a few others with whom to share it. Or you might instead share it with all your friends, or you might share it with the world.

The wording makes it sound like this is more an early test of a site than a core feature of Office, but seems to have a lot of potential.