The correct answer to "shelves in supermarkets in socialism were empty" is "that's because all people could afford to buy what was in them and there was no advertising need to overstock". Basically, the reason why supermarkets are overstocked is the fact that there are people rummaging through the trash searching for those very goods but outpriced.

If you run planned economies, you WANT shelves to get empty, shelves not getting empty means you're tossing away a significant part of your production, which is incredibly inefficient.