Well done for doing something about this. I know what you mean about those social media ads – they look so good but regularly aren’t!
In this case, as you have documented evidence of how bad the product is, together with the company’s refusal to refund you, I think it would be fine not to send them back.
It’s certainly a good idea to pursue them through your credit card company. Actually, you can also do Chargeback through the card-issuer, even if the item is worth less than £100. If you send them all the details and the proof of your contact with the company – particularly their refusal to accept blame – then I’m sure they will be able to get the money back.
If you have time, I would also complain to the social media company that they advertised on. They should know about it and if a lot of people complain then they will probably block the ads.