StumbleUpon is a great advance in the technology of procrastination. Install the add-on, and there's a button in your browser you can click to go to a random website that matches your interests, which are either stated explicitly or revealed from up/down votes on previous pages. I used to be an avid user (in my old blog posts, which often started with "I stumbled on...", it means I literally used StumbleUpon to find it). But no more.
1. It was killing my productivity.
2. Average quality of sites I found wasn't very high. It was taking 10-15 clicks to find a site I actually thought was worth reading (and occasional malware or browser exploits to spice things up). Using it was a quick path to carpal tunnel syndrome.
3. StumbleUpon helps content-farming websites, which re-publish copyrighted work and give no credit to the original authors.
Maybe you've heard about the recent row between The Oatmeal and humor-aggregation site FunnyJunk (which I will not be linking to, out of principle). To summarize: comics from The Oatmeal were appearing without attribution on FunnyJunk. Oatmeal writer creates blog post complaining about this fact. A year later, FunnyJunk sues him and demands $20,000 payment for harming their reputation. In response, Oatmeal sponsors a fundraising drive to help The American Cancer Society and Wildlife Foundation. FunnyJunk sues the charities as well. Brilliant legal work here.
The lame antics of FunnyJunk aren't really StumbleUpon's fault, but the stumble model of web traffic does help such content-farming sites disproportionately because they have a high link volume, allowing them to cash in on ad revenue and leave actual content creators out in the cold.
I personally have benefited from StumbleUpon traffic - a post from two years ago on the Fermi Paradox has gotten nearly as many clicks from StumbleUpon as the rest of my site combined - but personally, my days of stumbling around the web are over.