Like all social networks, the more you put into it, the more you get out of it. AND, the more self promotion you do, the less you'll get from it.
StumbleUpon is a platform discovery tool - its purpose is to put your content in front of users who have expressed an interest in your posts' categories. It primarily uses two signals for this - a simple checkbox system that a user manual chooses, and then the second is the way that user interacts with content they discover. It learns what you do and don't like through its thumbs up / thumbs down system.