Stumbleupon is now [mix.com](mix.com), which isn't the same but it's something. You used to be able to import old Stumbleupon links to the new site, it might still be possible. Other than that, there is r/internetisbeautiful which occasionally has some great new websites posted, there are many curation portals, mainly for articles, (apart from Pocket) like Refind, and I always encourage people to search for anything they're curious about. Searching your interests, even if you know everything about them, can lead you to some pretty niche websites. Google's algorithms have been a hit or miss lately but DuckDuckGo still seems to turn up proper results.
I know it's easier to simply scroll through feeds of the 4-5 regular sites you visit, but there still are a lot of interesting websites out there and many are making new ones, with the main struggle being getting people to their websites and engaging with it and not just seeing the screenshots on social media. The internet's economy is attention and we can all be a little more mindful of where we spend it.