I consider this spam. People favorite items for various reasons, and some of those reasons don't have the intention of buying the item.
I used to reach out to people if they liked my stuff, than I got put off when people started doing it to me. Than I heard it might be considered spam if we do it, and between the two, I just stopped.
I would err on the side of caution, and unless if customers speak to you first here or they actually ordered from you, that's the only way I will contact a customer.