Facebook CEO Mark Zuckerberg testifies before a House Energy and Commerce Committee hearing regarding the company’s use and protection of user data on Capitol Hill in Washington, April 11, 2018.

Facebook's product management director, David Baser, just wrote a blog post explaining when Facebook and its partners collect your personal information and when it's shared.

It also reiterates how it's able to gather information on people who don't use Facebook.

Here's the nugget you need to know about:

When does Facebook get data about people from other websites and apps?

Many websites and apps use Facebook services to make their content and ads more engaging and relevant. These services include: - Social plugins, such as our Like and Share buttons, which make other sites more social and help you share content on Facebook;

- Facebook Login, which lets you use your Facebook account to log into another website or app;

- Facebook Analytics, which helps websites and apps better understand how people use their services; and

- Facebook ads and measurement tools, which enable websites and apps to show ads from Facebook advertisers, to run their own ads on Facebook or elsewhere, and to understand the effectiveness of their ads. 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. (Emphasis added.)

That means Facebook collects data from a lot of places, such as apps that let you log in with a Facebook account, news sites that let you share articles to Facebook, and other spots.

It's really hard to avoid Facebook's reach.

Baser said Facebook collects information including your computer's IP address, the type of browser you're using to access the internet, the software your computer runs (Android, macOS, Windows, iOS, etc.), and other material.

Baser said Facebook doesn't sell that data but only uses it to cater content to you. While that's true, it can also use that data to target ads that it sells more accurately at its users, and can better understand what its users are doing online. "If you visit a lot of sports sites that use our services, you might see sports-related stories higher up in your News Feed," Baser explained.

This is one reason why it sometimes seems like Facebook ads are following you.

Most of this isn't new information, but it's part of Facebook's initiative to be more transparent with the government and its users about how the data it collects is shared. To limit the data Facebook can collect, follow CNBC's guide.