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.)