“/s” was popular on reddit (though not with everyone), probably because so many users didn’t speak English as their native language, and sarcasm could be hard for them to detect.

This is anecdotal but I believe that the /s tag was so common in Reddit for another reason: users (both native and non-native) unable to retrieve info from context. Those context-illiterate users miss the “clues” that a certain statement should not be taken at its face value.

I’m saying this for a few reasons:

• Decontextualisation is rampant in Reddit. Even within a single comment chain.
• I’ve informally observed a few discussions caused by poster A saying something clearly sarcastic, and poster B taking it at face value. A lot of the times, both A and B are native English speakers.
• When you’re speaking, tone provides a clue to interpret a statement as sarcastic. The same clue doesn’t exist when you’re writing.
• Even in communities primarily shared by monolinguals, you see similar resources popping up. e.g. Portuguese speakers using “rsrs” (risos = laughs) to highlight “I’m not saying this seriously”.
• Native English speakers were a small majority in the site, around 60% of the users. (Dunno how it is now - the exodus might’ve changed demographics.) I don’t think that non-native speakers were such a huge pressure on the behaviour of the site.
• Those failures to correctly interpret a comment as sarcastic were usually fairly consistent. That doesn’t fit well with the idea of a diverse non-native demographic.