Watch out when using tracking pixels. I am sorry I've not been able to find the source yet but in the past Facebook has said that they use them to target ads at

Watch out when using tracking pixels. I am sorry I've not been able to find the source yet but in the past Facebook has said that they use them to target ads at other users if they successfully hit your tracking pixel. It makes sense that they would as they now know that user is likely to click ads and follow through, but will in the long run increase your advertising costs as you're now competing against others for that user