working on a multi-user site

Hello! I am running a wordpress multi-site. There are many users who are with different rights and levels on the main site. I want that when users post something on the main site, they will also auto-post to their facebook wall. I tried before but posts from different users all go to one single facebook account (admin's). Is it possible to do it with many users on the main site?

If not, can I achieve something similar by assigning one blog to one user? How will users have to do so that any posts to their blog will be auto-posted to their distinct facebook wall?