I'd say stack overflow. They're both a relatively simplistic website showing text messages posted by users. Nothing fancy.
Reddit is a simple message board as there were many 15 years ago. They don't host images or video themselves, which avoid the issue of bandwidth. They don't seem to have the breath of analytics and advertising tools offered by google or facebook.
Edit: here's some confirmation.
Reddit has 330M active monthly users: https://www.socialmediatoday.com/news/reddit-now-has-as-many...
And SO has 10,002,577 total:
https://data.stackexchange.com/stackoverflow/query/edit/9766...
I wouldn't be surprised as well if reddit was two orders of magnitude more than that, but that's still a fairly moderate site to operate.
It is numerous orders of magnitudes away from the like of google, youtube or facebook.
Maybe things have changed, but in 2016 they were running three or four dozen servers in each of two data centers:
https://nickcraver.com/blog/2016/03/29/stack-overflow-the-ha...
You can see that most of the servers are deployed in pairs, the minimum to have any redundancy, so they are nowhere near capacity.
Another blog post stated that the servers rarely go above a few percents of usage, they are very powerful, often close to the highest specs available.
I will stand by my statement. It should be largely possible for reddit to serve 2 orders of magnitude more traffic than stack overflow with 1 order of magnitude its hardware. We're talking a couple of racks times ten.
You said a couple, which usually means 'two'.
You offered StackOverflow as "another website with Reddit's daily active users and revenue profile, but significantly higher availability" and then said this:
"I wouldn't be surprised as well if reddit was two orders of magnitude more than that"
> I will stand by my statement
I have no literally idea how to have a conversation with someone who's willing to revise their previous numbers by a factor of ten or more, say it's the same statement as it was before, and call it good.
Reddit raised hundreds of millions of dollars, whereas stack overflow didn't, they have the means to buy a rack more if that's what is missing. There is no excuse to be slow or broken all the time.
The amount of traffic is not significant. It's perfectly scalable and shardable, it's only text over HTTP and most of it is cachable by a CDN.