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.
We did at first, but then removed it after realizing it was just another obstacle in the way that doesn't really do anything to remove spam or anything since anyone can make a new email account in mere seconds.
Of course a lot of communities have strict rules against blatant self-promotion. Otherwise they would be overrun by spam and ruined.
But if you are there, as a professional/expert in a specific subject, and provide "value" and stay on topic, it's usually OK to drop a link once in a while (sometimes for needed context).
I simply try and follow the Reddit famous "10% self-promo" rule everywhere, not just on Reddit.
A colleague of mine recently wrote about exactly this.
Here's a few excerpts:
If your goal is to be heard, you need to fucking scream! We spend so much time crafting our thoughts into these prefect morsels of information, yet so little time finding an audience to read and acknowledge our work. The average blogger spends roughly 3.5 hours creating a blog post, yet only about 15 – 20 minutes promoting it.
It’s published! My work is over, it’s time to celebrate. Ugh, nope. It’s what you do after you hit that publish button that will set you apart from the rest.
Don’t believe the “if you build it, they will come” fallacy. It’s the single biggest reason most bloggers fail to secure an audience for their content and ignore the 80/20 principle. Having said that, finding an audience is no easy feat. In fact, most successful bloggers will argue it takes considerably more effort than the blog post itself. It’s a slow and steady process as each little plug here, a share there, a mention or a comment compounds the next until you start seeing serious results.