When your users come from different geographic locations, you cannot choose one optimal server location so that your online content can be fast for everyone. In this case, rather than putting your website content (photos, videos, files, etc.) a single server, you can distribute the files and load across multiple systems. It is when you need a CDN - Content Delivery Network.
When mentioning to a social network having lots of user content, CDN is always an important factor which makes your site faster. phpFox CDN integrates with lots of popular CDN services to allow your users' content be distributed & cached at the CDN sites for faster loading. phpFox also provides a native integration with Amazon CDN systems by utilizing S3 and CloudFront for file storage and content distribution.