What ever happened to slashdot.org?
I asked myself the same thing not too long ago and ended up putting slashdot back into my RSS reader for a while. I was (sort of) surprised to see that basically everything in the "best of" HN RSS feed was also covered in the slashdot feed. They were almost identical for the few weeks I was subscribing to both, the same stories are covered...
the cycle is continuous.
You do have imgur which at one point was largely teamed up with reddit and was originally built for reddit in the early days. However reddit got unhappy that they were making money off reddit’s user base so they quietly sank that ship.
Not sure where gfycat originated from but Digg was my first aggregator site before the makeover which I can only assume content came from HN too.
Perhaps more curation and digest solutions will be soon built for large chat networks on Discord, Slack, and Telegram. I haven't seen any good curation and digest solutions for those networks yet.
There aren't these kinds of incentives (nor clear rules as to what content is considered on or off topic) on any mainstream social networks, and the operators themselves would rather have an disorganised army of idiots wasting time spamming random crap (and clicking on ads) rather than having a community centred around a set of guidelines and a common goal. Like if I sign up on Facebook, Twitter or Instagram today there isn't a FAQ post to let me know what's the objective of the platform, what content should I post, etc. Worse, I can isolate myself in echo-chambers by following the wrong accounts (which the platform will encourage by recommending more accounts like those) and this is how you end up with misinformation, etc.
See also Twitter; it used to be for mostly tech people due to the relative complexity and unwritten rules of the platform acting as a gatekeeper preventing idiots from coming in. If you put in the time to learn and understand what the platform is about and go through those barriers then there's a good chance you'd be a productive member of the network, so overall it self-regulated quite well. Unfortunately, these users are also the least likely to click on cancer aka ads, and since big tech is allergic to letting people actually pay for a service they'd rather dumb down the platform and open the floodgates and the cesspool that is today's Twitter is the (predictable) result.
It's only because the very competent HN mods put their thumbs on the scales that it works out.