When you add your Facebook pixel ID into a new domain, you should verify this domain on Facebook.
If you are using a sub-domain, like social.company.com and if you domain company.com was already verified, you don't need to do the verification again.
Verifying your Domain on Facebook
1. Add your custom domain on PixelMe.
You should add your custom domain on PixelMe before verifying on Facebook. If you are looking at how to add a custom domain on PixelMe, please follow this link.
2. Add your domain on Facebook
1/ Go on your Facebook ad account by following this link.
2/ Select the business account to declare your domain.
3/ Click on the button "Add"
4/ Add your domain into the field and click on Add Domain
5/ Select the second tab: "HTML file upload" and click on the link in step 3.
6/ Click on Verify!
Your domain is now verified on Facebook and your pixel will be correctly fired from all PixelMe links.