"First Floss" aka "Flexing Flosser"
Standard white floss stick spotted in the parking lot of the Concourse Athletic Club, Atlanta, GA in February, 2012

While not the first flosser I've ever seen discarded on the ground, this one gets the honor of being the first post on WTF - Where's That Floss?!?  Why, no one asks?  Because it's the first one I ever took a picture of.  This was spotted outside of my old athletic club.  I can certainly understand why someone would want to clean his teeth on the way into the club: He munched a Peanut Butter Clif Bar on the drive up and wants to make sure he's looking fresh before chatting up one of the foxy yoga instructors.  I've been there, man.  But there are trash cans literally every 25 feet from the parking lot to the club's entrance.  Why just throw the flosser on the ground?  A mystery indeed.