>People post their photos on Facebook groups. They get attention, likes - all the stuff that matters to a human. A week later the photos are lost in the group, hard for anyone to find, no indexing, no exposure.

Not even a week if you consider the target audience for what's posted as opposed to the poster. Algorithmic sorting and infinity scrolling have pretty much eliminated the ability to go back and look at something you saw a few days ago (unless the algorithm decides to boost it back into your feed).