It's a way to raise engaging. You have to be always there, or nobody will read you.
Very likely.
I have the theory that at least some of the big sites got there by getting something right by mere accident which in theory it is wrong, but makes them really successful. Sometimes, people analyze it and understands it. Sometimes, not. So the day they try to fix it, they see stats going bad. So that something stays forever, without nobody knowing why, only that it works.
@jgg it's not a design per se; it's completely by accident. But you ain't wrong