There is controversy about this issue. Some evidence suggests that the sling counts as a thrown weapon for some things, and projectile weapon for others. There is other evidence that it is only a projectile weapon, and never counts as a thrown weapon. There have been huge threads about this issue, usually involving halflings, and weather they get their bonus to thrown weapons from using a sling (and sometimes also involving the halfling skip rock).
No matter how loud people scream from each side, and no matter how many rules they quote (usually with a paragraph that starts with "Clearly...", always a sure sign on these boards that what is to follow is anything but clear and certain), in my opinion this is an unresolved rules issue. Perhaps someday it will find its way into published Sage advice or the FAQ, and perhaps not.
So here is my best advice (well, it's advice someone else gave me at one point on this board). "There is no spoon." It doesn't matter what the "official" ruling on this issue might be. The rules are all 100% suggestions and guidance only.
So just decide what you want your sling to be in your game. There are good, solid arguments on both sides. You can view a sling as something that is more about the strength of the individual whipping the sling forward that matters, much like a javelin. You can also view a sling as something that is more about the "shot" or stone that is in the sling, and the tensile strength of the fabric of the sling, much like a bow and arrow. The former would use strength, the later wouldn't.