Well, you must admit the average age of persons using Facebook in the span of a few years went from university age to something like mid-30's or 40's. It's just not "cool" anymore.
I'm in my early 20s so I was on social networks well before those of my parent's generation. Just try dodging your parents' friend requests, that never goes well...
If the general population median is mid-30s or 40s then any service that gains users in the overall society will end up with a same-ish median.