I have been using stumbleupon since many days. One must note that the moment you submit your link the stumble upon servers from different location ping your website.
That should not be considered as actual traffic. The more links you submit from your blog in a particular day those many visits you might find..so if you submit 5 links you would probably get 5 * 3 = approx 15 automated visits.
I do get legitimate visits from stumbleupon but the frequency is very moderate.
http://www.stumbleupon.com/stumbler/harshads83/
Continue stumbling.. guys..
Yes I noticed that too and always wondered how come on submitting I get such huge traffic instantaneously...
http://www.stumbleupon.com/stumbler/pramodlohia/
following,
True.