How can a large app keep adding major features without breaking everything?

Think of the app as a shopping center. Your account is the membership card, while messaging, video, profiles, and the marketplace are separate stores. The overall interface makes them feel like one product, but each feature can call its own backend services and use shared account or identity services. That separation also makes it possible to take one feature offline without necessarily bringing down everything else.

It’s also worth correcting the starting point: Facebook wasn’t primarily a chat app. Its early focus was user profiles and social connections. Messaging and the other major products were added over time, often by building separate systems around shared identity and platform infrastructure rather than turning the original codebase into one giant feature.

Interfaces are the key idea here, especially at a larger scale. A service can be implemented or changed internally as long as it continues to provide the same expected inputs and outputs. This lets teams work on different areas independently and replace components without forcing the whole application to be rewritten at once.

There isn’t a perfect architecture that lets a company add anything forever without changing old code. Large companies do lots of maintenance, migrations, refactoring, and sometimes major rewrites. Good design reduces how much a new feature affects existing systems, but business requirements and scale eventually force teams to change earlier decisions.

The features usually aren’t stacked into one enormous block of code. They’re built as separate modules or services that communicate through well-defined interfaces. Shared systems—such as accounts, authentication, permissions, notifications, and payments—can be reused by multiple features while each product area keeps its own logic relatively independent.