Why do big websites (e.g., Facebook) use a separate domain to host their resources? : webdev
This explains why major sites like Facebook use separate domains to serve static content: sending cookies with every HTTP request wastes significant bandwidth, especially on mobile connections where upload speeds are slow relative to download speeds. The cookieless domain approach lets sites avoid transmitting cookies for requests that don't need them, while CDNs provide additional benefits like serving content from geographically closer servers and letting browsers cache popular libraries across multiple websites.