When you submit to reddit choose the appropriate category for your post. I've also submitted to reddit articles and urls and got traffic but not a huge amount. You have to select the proper category. May be your post wasn't submitted on the right category or your post is not in a search. There are lots of competitors and if you have submitted a post where there are a huge amount of competitors it would be hard for people to find yours. So it's very important when you submit to choose the best category where you can get traffic. And you have to be active too. Just read others articles, posts and comment on them. Try to get found as a user where you can be able to get found for your posts too.
SU is like a high speed superhighway, the next new thing comes along nano seconds later to divert attention away from your stumbled page/site.
For me, StumbleUpon and Pinterest are just the same. They don't bring much traffic. However, if your niche is all about women and fashion, Pinterest is surely a big help.
Yeah I get some nice traffic from StumbleUpon. I got 10k visitors in a day before with a lucky link. It also helps if you add a share button to your site so more of your content will be shared by your users to StumbleUpon. I also get some nice traffic from reddit in a niche subreddit.
There is no point of concentrating on just stumble upon. Use other websites as well. And if you are looking for traffic for your website, then you need to also use the whole of SEO techniques. Using just Stumble upon is like using just 1 percent of the SEO methods.