I trace with my fingers too!! All the time! It's kind of compulsive seeing as I do it mostly all day no matter what I'm doing or what surface its on be it my arm or a table. I'm 17 and have done it since the 3rd grade. I hardly even notice until people ask what I'm doing.
Haha, just one of those little habits. I also sniff my drinking cups?
That's another story though... :]