It's true that many people seem content to get a message on a social media site acknowledging their birthday. However there is still a market for cards, I sell cards almost on a daily basis. However, I would suggest you expand to other areas as well. If you have created a digital design consider placing that on other products. You've already done the hard part designing it.
If it is a physical product, could you turn it into a wall hanging, apron, or pillow covering as examples. Many plain items such as aprons, t-shirts, or pillows can be purchased in bulk cheaply for you to adorn with your design. These then could be sold at craft's fairs or via sites such as Etsy.