As with any social media site, you have to be careful about only sharing your own work. You have to share other interesting things in-between. With StumbleUpon, it's as simple as stumbling other pages, choosing some pages you like, and adding some links that aren't yours. I have been using it that way for more than a year now with no problems.
Other than that, only submitting your own work is viewed as spam. You have to make your posts anywhere interesting and of value to all other users. You may not ever find a site where it's acceptable to only post a whole bunch of your own links and nothing else, unless it's your own blog or personal website.