Facebook alone is far more mainstream than Reddit, I would say. Thousands of times more.
Maybe ten years ago, but that's a huge exaggeration today.
Roughly 6x the users and 7x the traffic. “Thousands” is a bit dramatic. Reddit is a massively popular site now, pretty sure it’s among the top 10 most visited in the world.
We can just look at both's recent reports:
Reddit: 121 million "daily active unique users"
Source: https://investor.redditinc.com/news-events/news-releases/new...
Facebook: 3.58 billion "family daily active people"
Source: https://investor.atmeta.com/investor-news/press-release-deta...
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I'd definitely consider both "mainstream".
(I know plenty non-tech and "normal" people who use reddit.)