My team and I run Snapzu (
http://www.snapzu.com), a which we started several years ago.
First off, the main reason there's no de facto "Reddit alternative" (Like Reddit was to Digg) is because it's nearly impossible to grow a community based around thousands of topics from scratch.
It's a massive "chicken or egg" dilemma where you need a steady flow of users (1000's per day or more) to provide the necessary community "value" to other users. And even then, the users are scattered all over the place with all kinds of different interests, making it hard to keep them around and coming back for more.
I've kept an eye out on all the competition that has come and gone (hundreds of them at this point), and most of them don't make it even one year before throwing in the towel.
Building a link-sharing/discussion platform is one thing (anyone can do it), but populating it with active people is a whole other beast and probably 100x harder, requiring routinely time consuming tasks such as blogging, promoting the blogging, and trying anything and everything to get the word out. This is where most people fail, and the saying "build it and they will come" is utter BS.
It also doesn't help that Reddit has copied (probably unintentionally) many of the things we added over the years to try and differentiate from them. We had their new "cross-posting" feature many years ago, but ours is named "mirroring" where you can add an existing post into other tribes (our version of sub-reddits). We also had the ability to add "related links" for years now, and I've noticed they added that feature to their "mega-thread" topics, and I'm sure they will make it a site wide feature soon enough.
Reddit gets millions of daily visitors from SEO alone, and all of that SEO is dependent on their user-generated content. For MANY search terms, you can find a Reddit link on the first page of the Google results.
At this point we've had to pivot (focusing on bloggers/creators and their audiences) because we understand that we can't directly compete with them.