Azure Front Door is the more common of the two in our index: it runs on 505 websites against 357 for Amazon ELB. Counts come from real scans, not vendor claims.
AWS ELB is a network load balancer service provided by Amazon Web Services for distributing traffic across multiple targets, such as Amazon EC2 instances, containers, IP addresses, and Lambda functions.
Azure Front Door is a scalable and secure entry point for fast delivery of your global web applications.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.