One way would be to setup a Success Event to count Favourites. You can set this Success Event within s.products level (so that it is associated with each product) or at the hit/beacon level (so that it is associated with all items in s.products).
So every time the user Favourites a product, you send a Custom Link beacon that includes s.products (which would would have the product name (or whatever you're using to track products in there)) together with the Success Event (within s.products (if setting it at the product level) or s.events (if setting it at the hit/beacon level)).