DocJohn explained it a while back; the "number" where you can't see any names is people who have added you as a friend. I have "27". So, 27 people like me :-)

It is different from those whom I've added, that's my own friends list (I think I have 4 or 5) so when I send a PM, I can just click on their name in the drop-down box and don't have to type it in. Their name and avatar shows up when I'm in my PM box and they're online so I know I can "reach" them right then and say, "Hey, what you doing?" and hope for a reply :-)