I do it. If they have bought from you, they'll be happy to see ways to keep in touch that they may have missed. As a buyer, if there's company/product/event that I'm interested in and it has a FB page, I follow it to keep in touch and keep up with news and promotions. As a seller, I think the "thank you for your purchase" is a good place to do that. You know they're excited enough about you to make the purchase, and they're more likely to want to keep in touch, giving you a way gain those "true" FB fans.