Except for Announcements, none of the course content is pulled out of the Blackboard system. Facebook users just receive notifications about each new or updated piece of information, complete with a direct link to the item in their Blackboard course. So for example, if there is a new grade posted, Blackboard Sync will notify the student in Facebook that there is a new grade posted, and provide a link to their View My Grades page in the Blackboard Learning System. The student will still need to go to Blackboard to see that grade. Similarly, assignments and other course materials, forum posts, etc. are not transmitted to the Blackboard Sync application in Facebook, just notifications about them.
The information which is transmitted over the Internet - the announcements, notification information, course content tree, etc. is encrypted until delivery to Blackboard Sync.
Additionally, each student can only see information specific to them for their own courses. They can't share that information with Facebook Friends or Networks and there is no way for other students to access their information through Blackboard Sync.