I have had decent success too.

From my personal experience; even if you are using these auto *submbling and dig*ging tools, the key is to find unique interesting content. Also the niche of the post matters; something I noticed is:

dig*g: more targeted towards tech enthusists
stumbl*upon: more targeted towards people trying to find funny/interesting stories, games, cool stuff

People actually do comment on your postings, and that will increase the value of your submission.

hope this helps.