A lot of reddit and twitter posts are already just people regurgitating and rehashing what they have heard/read from others, mostly without a lot of critical thought. Is this really going to be very different?
Beyond these text-based social media platforms, I could see the next generation of TikTok/Instagram "influencer" social media adopting AI rails so there's next to zero effort put into creation. The photography, trends, personalities - all of it - will be generated.
The above was also expressed (in less cruel terms) by DFW in his "what is water" commencement speech at Kenyon College: (again, from memory) it is our natural tendency to operate in "default mode", which we must fight.