Arguably reddit isn’t even mainstream ...
... with just 0.91% of US social media visits this year in March this year, if this isn't wrong:
https://www.statista.com/statistics/265773/market-share-of-the-most-popular-social-media-websites-in-the-us/
FB 53.09%, Twit 16.25%, IG 13.85%, ..., Reddit 0.91% ...
[Edited to fix my error.]
[I have no affiliation with the linked site.]