Why Facebook Has Stuck With PHP

PHP has a long history with Facebook, and it was the primary programming language used to build the site in its early years. Facebook has spent a significant amount of time and money developing tools and infrastructure to support PHP at scale.

One of the primary reasons Facebook continues to use PHP is its ability to optimise and scale PHP to meet its needs. Facebook has created its own PHP engine, HipHop Virtual Machine (HHVM), which is optimised for quickly and efficiently running PHP code. This has enabled Facebook to handle its platform’s massive traffic and data demands.
Another reason Facebook keeps using PHP is that it has a large highly skilled PHP development team. Facebook has invested in PHP developer training and has created a culture around the language. The company has also given back to the PHP community by releasing open-source projects like HHVM and Hack, which is a programming language built on top of PHP.

Finally, Facebook has built a massive codebase in PHP over the years, and switching to another language would be a significant undertaking. To migrate all of its existing code to a new language, a significant investment in time, resources, and training would be required.
To summarise, Facebook continues to use PHP because it has heavily invested in the language and developed tools and infrastructure to optimise and scale it. It also has a large and experienced PHP development team, as well as a large codebase in the language. While other languages may have advantages, the cost and effort required to switch to a new language would be significant.