Facebook faced several challenges with their web development process, primarily related to the complexity and unpredictability of their codebase.

Here's a breakdown of the issues:

• MVC's limitations: As Facebook's applications grew, their traditional Model-View-Controller (MVC) architecture became unwieldy. Adding new features led to an explosion of dependencies, making the codebase increasingly difficult to understand and maintain.
• Unpredictable code: This complexity made the code unpredictable. Engineers struggled to grasp the impact of changes, leading to bugs and slow development cycles.
• Fragile UI rendering: The rendering layer for user interfaces, which relied on imperative DOM manipulation, was fragile and inefficient. It became difficult to handle complex UI updates and maintain performance.
• User frustration: These issues culminated in a frustrating user experience, particularly with the chat feature, where bugs and inconsistencies plagued the product. Users expressed their dissatisfaction loudly.

To address these challenges, Facebook created React. React is a JavaScript library designed to tackle the complexities of building dynamic user interfaces. It introduces the concept of a virtual DOM, a lightweight representation of the user interface, and a declarative approach to rendering.

Here's how React helps solve Facebook's problems:

• Predictability: React's declarative approach makes code more predictable. Developers describe what the UI should look like at any given time, and React handles the necessary DOM updates, eliminating the need for complex imperative manipulations.
• Performance: The virtual DOM efficiently differs changes, only updating the DOM elements that have actually changed, leading to significant performance improvements.
• Maintainability: React components are reusable, making it easier to maintain and extend the codebase. The single directional data flow in React also makes it simpler to understand how data flows through the application.
• Improved user experience: By streamlining the development process and enhancing performance, React ultimately contributes to a better user experience, addressing the frustration that plagued Facebook's users.

In essence, React allowed Facebook to build more maintainable, performant, and user-friendly web applications, ultimately addressing the core challenges they faced with their previous development approach.

Resources: https://www.youtube.com/watch?v=nYkdrAPrdcw