Even though ad-blockers are ever more common ? (I didn't even think about ads, since I never saw them on imgur !)
It's the same way Google pulled in $180B last year and Facebook made $86B.
AFAIK both of these are using methods that are either deemed acceptable by adblockers (like how Google was using plain blue/black text instead of stroboscopic gif banners), and/or adblockers have trouble with them because they come from the same source ?
They have an app which is ad supported, and they are trying to become of a social network like reddit by requiring you to login for more features, and adding upvotes and other interactivity.
Low cost CDNs like Cloudflare.
Imgur uses Fastly, which I don't think can be classified as low cost.