I can see how that kinda makes sense.

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.