After Facebook CEO Mark Zuckerberg testified in front of the US Congress last week, the company on Tuesday tried to clarify on questions how it collects data when people are not directly using the website or app.
Many websites and apps use Facebook services to make their content and ads more engaging and relevant.
"Apps and websites that use our services, such as the Like button or Facebook Analytics, send us information to make their content and ads better," David Baser, Product Management Director at Facebook, wrote in a blog post.
In return for that information, Facebook helps those websites serve up relevant ads or receive analytics that help them understand how people use their services.
"When you visit a site or app that uses our services, we receive information even if you're logged out or don't have a Facebook account. This is because other apps and sites don't know who is using Facebook," Baser added.
Many companies offer these types of services and, like Facebook, they also get information from the apps and sites that use them.
"Twitter, Pinterest and LinkedIn all have similar Like and Share buttons to help people share things on their services. Google has a popular analytics service. And Amazon, Google and Twitter all offer login features," Facebook said.
These companies -- and many others -- also offer advertising services.
When you visit a website, your browser (for example Chrome, Safari or Firefox) sends a request to the site's server.
The browser shares your IP address so the website knows where on the Internet to send the site content.
The website also gets information about the browser and operating system (for example Android or Windows) you're using because not all browsers and devices support the same features.
"It also gets cookies, which are identifiers that websites use to know if you've visited before. This can help with things like saving items in your shopping cart," Facebook explained.
"So when a website uses one of our services, your browser sends the same kind of information to Facebook as the website receives. We also get information about which website or app you're using, which is necessary to know when to provide our tools," Baser noted.
There are three main ways in which Facebook uses the information it gets from other websites and apps.
"Providing our services to these sites or apps; improving safety and security on Facebook; and enhancing our own products and services," Baser said.
"We also use the information we receive from websites and apps to help protect the security of Facebook. For example, receiving data about the sites a particular browser has visited can help us identify bad actors," he posted.
Zuckerberg, appearing before the US Congress last week, told the lawmakers that his own personal data was part of 87 million users' that was "improperly shared" with British political consultancy firm Cambridge Analytica.
"We don't sell the data. We use the data that people put into the system in order to make them more relevant. I believe people own their content," he told the US Congress.
Facebook is embroiled in a widening scandal that a British data firm called Cambridge Analytica improperly gathered detailed information on its 87 million users.