StumbleUpon is a great content marketing platform when done right: Just "bookmarking" your own pages there won't have any results. It all comes down to sharing great content and helping others. Building a popular quality profile there may take up to a year.
I have a blog where 70% of traffic comes from SU and 90% of that traffic goes to one blog post that did particularly well. It has been generating that traffic for the past three years.
How good that traffic is? It's good enough to keep growing the readership of that blog (subscribers, etc)
Can you do the same with the project page on SU? I doubt so, unless you have a very fun project.
That being said, SU should be part of content marketing plan but it's a long-term time investment to make it work
Everything will be ok in the end
If it's not ok, it's not the end