If you’ve been looking at your Google Analytics data (which I know you are), you’ve probably seen traffic coming from “l.facebook.com”. You might be asking yourself, “What the heck is that?”
Well, it is traffic coming through something called a “link shim”.
Link shims were created by Facebook to protect you from malicious links. When you click on a link in Facebook, it checks that link against a huge list of malicious URLs and, if there is any concern, it will ask you if you want to proceed to the URL you clicked. That is the link shim.
So, that is desktop traffic coming from Facebook simply separated out in your Google Analytics because it is coming through the link shim. If you’re interested to delve deeper into this topic, here is more detailed information.