Is it permissible to fund Facebook ads using a loophole that allows for obtaining free money, and then profit from this funding?

If the method of adding credit to your account involves deceiving the website, or hacking, or fraud, or exploiting technical errors, then this credit is forbidden and must be returned. However, if the method is in accordance with the website's system and free from fraud, then there is no harm in benefiting from the added credit, and there is no harm in selling advertising credit for more than its value.