Without doubt stumbleupon does send traffic and produces and extremely high bounce rate that queers your stats. Bounce rate is the percentage of visitors who enter and exit on the same page without visiting any other pages on the same site. Consequently visitors reading your post on the front page without clicking to open the comments is a bounce.
It’s important to comprehend that visitors who bounce out of a site immediately after arrival do ratchet up the unique visitor count, but do not make no contribution to long-term value. That’s why a high bounce rate is not desirable.
If you are into AU then do be sure to watch your metrics and do understand that an influx of one hit wonders who produce a high bounce rate are not a true indication of the popularity of either your posts or your site. We bloggers are aimed at converting first time visitors to becoming regulars who also comment.
Also many newbies to stumbleupon think they can get away with stumbling their own posts or repeatedly stumbling posts from the same sites when this is not the case. Everything we do at SU is tracked and the site is not meant to be used in that manner. So do be aware that if you adopt that behavior the ban hammer will fall on you http://sem-group.net/search-engine-optimization-blog/when-the-ban-hammer-comes-down-on-stumbleupon/