What’s old is new again. In the 90s we used services like Submit It to get an URL into all the crawlers and indices. Now the search engines aren’t the challenge, it’s the sites targeting specific audiences.
Speaking of StumbleUpon, I'm not sure whether this was just luck or something about its recommendation algorithm/social graph, but it was the only service where I didn't see the usual flood of traffic followed by rapid decay, the classic Slashdot/HN effect. The curve felt much smoother.
I remember some bloggers at the time describing the same thing [1].
We used StumbleUpon to visit interesting sites we wouldn’t otherwise find. It didn’t exist to keep you deeply engaged with a main StumbleUpon website.
The aggregators are meant to be the destination. The links are more like shiny dangling lures. Some of them (reddit) do everything they can to keep you from having a reason to leave the page at all.
So I suppose it would follow that one gets people engaged in your site, while the other kinda tries to keep them from doing that.