You're making a great point about Facebook. It's still insanely popular among older and more rural Americans. I guess these are populations that would be more isolated without it and still see the value in participating. One example: I have a 21 year old niece who is on it 24/7, but then again she is a hour's drive from the nearest Walmart and and her parents. Three hundred miles from her bestie. She's two-thousand-ish miles away from me. She has my number, but she prefers Messenger to text.
It's also good for community groups, online garage sales, in memoriam pages, and the like. Some businesses like to use it instead of having a proper website, probably due to zero cost of hosting and familiarity of use.*
Reddit never fit naturally with any of those, but I'm sure it will find a niche of users in the same way.
*I'm sure there are other dynamics worldwide where Facebook might also be a valuable tool. These are just the first few examples I can think of in the US.