Facebook answers some questions Mark Zuckerberg didn’t in testimony

Mark Zuckerberg spent much of his congressional testimony making the case that Facebook Inc. users have control over their data. But what about the data the social network collects from people who don't even have accounts?

In Congress last week, Zuckerberg said Facebook needed to track people who weren't members for "security purposes." In a follow-up blog post Monday, the company laid out plenty of business reasons to do it, too.

Facebook said other websites and apps use its services to let people quickly create accounts and later share content on their Facebook pages, like news articles. Behind the scenes, Facebook also helps some sites and apps track their performance and run ads from Facebook advertisers.

Those sites send back information to Facebook on the people who visit them -- with details on everyone, not just Facebook users. "We receive information even if you're logged out or don't have a Facebook account," David Baser, product management director, wrote in the company's blog. "This is because other apps and sites don't know who is using Facebook."

The company is working to be more transparent after Zuckerberg's two days of testimony in Congress last week, during which the chief executive officer frequently deflected questions about how Facebook tracks its users, explaining that everyone has control over their own data.

Facebook said it needs the information in order to keep tabs on who saw which ads, to understand what promotions they might be interested in seeing next -- on Facebook or elsewhere.

"Cookies and device identifiers help us determine whether the person uses Facebook," the company said. "If they do, we can use the fact that they visited a business's site or app to show them an ad from that business - or a similar one — back on Facebook. If they don't, we can show an ad encouraging them to sign up for Facebook."

The company said it's not alone in tracking people around various apps and websites, and named several other internet services that receive information from sites they don't run.

"Twitter, Pinterest and LinkedIn all have similar Like and Share buttons to help people share things on their services," Facebook wrote in the post. "Google has a popular analytics service. And Amazon, Google and Twitter all offer login features. These companies — and many others — also offer advertising services. In fact, most websites and apps send the same information to multiple companies each time you visit them."ALSO READ: How to manage and block apps from accessing your Facebook data