Wow! This is unfortunate because that person has now been primed for that gift. If it's a fairly obscure product that it was possible they hadn't heard of, then they may not be as surprised upon receiving it, and either way if they are able to remember they saw it while you were browsing facebook, they may be less convinced of the genuineness of the gift, thinking you just clicked on whatever ad jumped out at you rather than using your creativity to select something.