Facebook turns 15 next month. When I started Facebook, I wasn’t trying to build a global company. I realized you could find almost anything on the internet—music, books, information—except the thing that matters most: people. So I built a service people could use to connect and learn about each other. Over the years, billions have found this useful, and we’ve built more services that people around the world love and use every day.

Recently I’ve heard many questions about our business model, so I want to explain the principles of how we operate.

I believe everyone should have a voice and be able to connect. If we’re committed to serving everyone, then we need a service that is affordable to everyone. The best way to do that is to offer services for free, which ads enable us to do.

People consistently tell us that if they’re going to see ads, they want them to be relevant. That means we need to understand their interests. So based on what pages people like, what they click on, and other signals, we create categories—for example, people who like pages about gardening and live in Spain—and then charge advertisers to show ads to that category. Although advertising to specific groups existed well before the internet, online advertising allows much more precise targeting and therefore more-relevant ads.

The internet also allows far greater transparency and control over what ads you see than TV, radio or print. On Facebook, you have control over what information we use to show you ads, and you can block any advertiser from reaching you. You can find out why you’re seeing an ad and change your preferences to get ads you’re interested in. And you can use our transparency tools to see every different ad an advertiser is showing to anyone else.