So for image sharing you have Flickr, and for music/videos you have Youtube for sharing.Both are these services are well done, have big money backers, and are household names. That leaves document sharing opening, sure there is Scribd & Google Docs but I would argue neither are house hold names.

You could also change up your game plan to offer something like Posterous. It seems like you've already created a service that easily allows users to get content up on the web. Maybe it's just a matter of giving them additional tools to share that content?