HTTP was designed for multi-page applications/websites but I think it makes sense that Facebook migrated from basic PHP/MySQL to something more complex includin

HTTP was designed for multi-page applications/websites but I think it makes sense that Facebook migrated from basic PHP/MySQL to something more complex including React and making less full-page navigation events, for performance reasons (server costs and mobile loading time). There's also UX because a PHP website can feel a bit static, with less animations. But yes the web has evolved since then and the HTTP protocol has newer versions and most apps don't serve as many users as Facebook.