The use of tracking pixels, such as Facebook and Google, are common ways to monitor how visitors interact with your website. Below is an issue that is often found with some implementations of this code.
Pixel image missing alt
Issue as it appears in DubBot (it may or may not display in current view):
If it does display you will typically find it near the top or bottom of the page.
A tracking pixel <img/> element is set within a <noscript /> element.
Adding alt="" will remove this issue by labeling the image as decorative.