I've have noticed that too, especially on other websites (not the social media ones). When I'm on, little to no messages. Probably because its a school or work week. When I'm not on, message box is pretty full (weekends mostly). I go to sleep, wake up and check on my messages. And there are like a bunch messages from overnight most of them from very early in the morning. But its all good knowing that there are lots of people out there that care about you.