According to Reddit's source code they're using a provider called Imgix - https://www.imgix.com/ - who I assume use S3.
What advantages? If you're running these as headless servers, then what advantages does OSX possibly provide over cheaper, more powerful Linux boxes using e.g. ImageMagick (ok, ok, not having massive remote code execution vulnerabilities is one of them).
(Guessing they use the fastly private cdn or have some sort of deal with them considering the imgix branded cages on their blog).