[macandcheese]

"a sort of low-effort stream of consciousness between friends."

That's exactly the draw for many people, and why it's so sticky. It's about as close to a genuine in-person conversation as you can get. I don't think of it as low quality, just low reach. I speak more colloquially around my friends than I do my employer, and I like Snapchat because it similarly allows me to communicate "at ease".