Definitely agree with you on how Reddit and forums are different, but they can still be pretty addictive. I just love reading discussions about stuff, and it really scratches an itch that is not satisfied by social media that is centered around personalities. I hate all the self promotion on sites like Twitter and Instagram anyway, as despite there being a lot of cool stuff, there are so many "influencers" and grifters out there it's honestly a massive turnoff to use the sites.
Add in that you can just check how the algorithm that sorts your posts works. Social media like Facebook have always felt like 100% ads with how content is sorted. If not for anotherproduct, an ad for Facebook itself.
It's different, in my opinion. You're not trying to expose yourself, you're not gonna change yourself so the others can think you're something you're not.