Consider your tires, always wearing down, aren't they? Every mile takes minute amounts of rubber off your tire. So what happens to that rubber? For an example, lets take the section of I-80 from Sacramento to Davis. Thousands of vehicles travel this road every day. A perfect testing ground for possible answers as to what happens to the rubber.
Let's go with the most obvious solution: it stays on the road. If that were the case, each lane would have two twin mounds of rubber on the usual wheel paths. Does it vaporize off the tire to to friction heat? No, our skies would be totally murky if that were the case.
The key lies in the memory of rubber, the inherent tendency for rubber to return to its original shape. As an experiment, go and get a rubber eraser, one of the pink ones will do best. Back? Good. Flex it a bit, and let go. It will spring back to its more molecules, until such time as it is moved by wind, rain or cars. Usually moved by cars, it will bounce along the freeway for some time, until it comes to rest on the side of the road. There it will attract other chunks.
This process continues until CalTrans comes by to clean the sides of the road, or until an entire tire is formed. Full tire regeneration is a painfully slow process and is extremely rare, but some documented cases do exist. This also is why you see so few blowouts, but so many huge flaps of tire on the roadways.