Simplest explanation is that the general public doesn’t give a shit and while Facebook is on the downturn (not sure if numbers can back that up) people need to go somewhere else. Maybe that is reddit right now, they got the marketing and content to get people on it.

I’d like to hear that crapbook is going downhill, but its share price says otherwise

Its share price doesn’t matter in this context, since Meta also owns Instagram, which is absolutely not going downhill at the moment.

Facebook however is losing active users, especially in the younger age ranges and even more pronounced in Europe and the US.