Nothing in the world is really free. These online storage companies offer you free space. That’s sounds good! But: there is a chance that you see some advertising during the time you are logged in or uploading your files. Not all services show advertiments off course.
Take into mind that advertising makes it possible to offer free online storagespace for you. In other cases a free backup account is way to attract more potential customers. They offer you a few GB’s storagespace for free, but these companies hope that you will upgrade to a paid account with more space and features.