It is not the Coke in your can evaporating, but water within the cola product. Water is a volatile substance that reacts to its environment, and takes many different forms that depend on surrounding temperature and pressure.
At normal conditions, water slowly evaporates into the air. In a sealed environment, it will evaporate until whatever air is available reaches saturation. Since cans of cola are not filled to the brim, some water will engage in this process.
As pressure builds during saturation, other water molecules condense back into liquid form (dew) until equilibrium is achieved. Water will continue to rise and fall in this manner in any environment or container -- it's why you see dew drops on the top of your old leftover boxes and why a piece of moist bread keeps a tin of cookies from hardening.
Conclusion: It's the Environment
While part of your cola product may be evaporating into the air around it, it shouldn't be escaping unless the environment is not entirely secure. Manufacturers now often label cans with "use by" dates because they make cans out of increasingly thin aluminum and seals that don't last forever. If left on a shelf long enough, the liquid in your soda will eventually start to evaporate and go flat.