Stumbleupon is meant to be used to discover and share new sites. It's not meant to be used by groups of friends who keep giving thumbs up to every post that's published on the same sites over and over again. It's not meant to be used by yourself to promote your own site(s) either. All activity on the site and by use of the toolbar is tracked very closely.
Technically you can stumble your own posts from time to time but not frequently. If you start up doing this frequently you will risk being either ghost banned or actually banned. Likewise if you tend to stumble posts on the same sites repeatedly.
Please see here for a relevant link > http://en.forums.wordpress.com/topic/noticeable-drop-in-traffic/page/2?replies=44#post-807990