Do a lot of people actually use it? It seems like you could just as easily replace that site by simple tweets when you actually stumble upon something worthy. Which is what most people do IMO.
Based on some very perfunctory research: they certainly used to have a lot of users, but I get the feeling they have declined over the past ~5 years.
Anecdotally: I used to use StumbleUpon, and found it fun for a while. I'm not sure what caused me to drift away from it, but the reason I have no interest now is primarily that I am drowning in interesting content. (I have enough articles saved to Pocket to keep me going for years.) Getting recommendations tailored to your interests is nice, but unless the personalisation algorithm is really good, I think you can do better by cultivating your own set of sources and intermediaries.
