You may not use LinkedIn, but if you've ever visited Wikipedia, Tumblr, or Twitter—or done anything while using the Mozilla Firefox browser—you've benefited from LinkedIn's code.

LinkedIn engineers developed and continue to steward the Apache Kafka messaging program, which just so happens to apply to a lot of services outside of itself. And since it's free and open source, companies like Twitter, Tumblr, Netflix and Pinterest use Apache Kafka, too.

It might surprise you just how often large companies give away their code to the open source community. At that point, anyone can contribute, debug, or download the code from a repository on GitHub or the Apache Software Foundation—and then use it for themselves. 

But how could companies possibly benefit from open source? Aren't they worried about giving away their "secret sauce?" And why aren't they spending their time focusing on the core product that will actually make them money?

I asked some of the biggest players in open source about why they do what they do. 

Square

With fewer than 700 employees, payments service Square is the smallest company on this list. But given its size, its contributions are especially notable.

Square CTO Bob Lee estimates his company has given a quarter million lines of code to the open source community through more than 60 Square-headed open source projects. 

Its most high profile offering? Picasso, an image downloading and caching library for Android now used by the New York Times, PayPal, Ouya, Spotify, and many more

Picasso demo on an Android phone. Picasso demo on an Android phone.

“Picasso solved the problem [of image caching] once, so people don’t have to keep solving it over and over again,” Lee said. “Since it’s under active development, people are working on it and improving it all the time.”

According to Lee, it’s because of Square’s small stature that open source makes sense for the company. Instead of throwing lots of internal resources at a general problems many companies face, it makes more sense to display Square’s tentative solution to the public domain and work with outside developers to build it. 

“It’s a really great way for us to pool resources with other companies like ourselves,” Lee said. “It frees up to build our product and spend less time on general purpose infrastructure.”

Google 

Google doesn’t head as many active open source projects as the other companies on this list, but it makes up for it in quality and size. For example, Android and Chrome are each listed as single projects, but they're both pretty massive. And development on their plugins and apps is nearly endless.

Square’s Bob Lee also noted that Google’s sheer numbers can turn the tide on another group’s project, like Square’s Dagger, a dependency injector for Android and Java.

“Now Google contributes to Dagger more than we do, so it’s like getting free software,” he said. 

For the insanely profitable Google, contributing to open source is less about acquiring resources and primarily about community building, according to Christopher Katsaros, the communications manager for Google’s open source Android project. Google’s Summer of Code, a program that rewards college students for contributing to open source projects, has contributed more than 50 million lines of code to the open source community.

“One of Google’s missions is to make the Internet available to as many people as possible," Katsaros said. "If ambitious projects like Android and Chrome have given users more access, that’s certainly in line with our goals." 

LinkedIn

Thanks to powerhouse data infrastructure projects like Apache Kafka, LinkedIn has contributed more than half a million lines of code to the open source community through roughly 80 different projects. What keeps them going? According to LinkedIn's principal staff engineer Jay Kreps, it’s about keeping standards high by making sure everyone’s eyes are on them.

“To encourage excellence, you need to do things out in the open,” Kreps said. “Engineers are like everyone else—if everybody’s watching, they want to look good. Otherwise we’re building a crappy internal tool that’s just good enough to meet our immediate needs.”

All of the tools LinkedIn uses internally are out in the open for anybody to work on. If it’s not directly a part of the company’s business plan, you can feel free to examine the code. 

“It’s not an effective strategy to have everything be our secret sauce,” Kreps said. “Internal messaging doesn’t need to be our competitive edge.”

Fittingly, this career network has found this open source strategy to be the best recruitment technique possible. 

“[Open source is] how you demonstrate in a way that is provable to people that we’re a good company,” Kreps said. “They can see every line of code and contribution. A lot of the people we’ve hired have said this takes a lot of questions out of coming to our organization.” 

Facebook

My ReadWrite colleague Matt Asay recently named Facebook the largest open source company of all, surpassing even Linux steward Red Hat.

“Facebook is built on open source from top to bottom, and could not exist without it,” wrote software engineer Jordan DeLong when the company released open source library Folly. That’s probably why Facebook has given back so widely with a variety of open source tools.

Server designs are just one component of Facebook's Open Compute Project. Server designs are just one component of Facebook's Open Compute Project.

What takes Facebook above the average contributor is that it's the only company to have open sourced an entire data center. Open Compute is an enormous project that makes every component of your average data center totally transparent, from power systems to servers.

Facebook is also one of the companies that has been contributing to the open source the longest. The company has been talking about the importance of its “open source ethos” since December 2006. So even if there were numbers on how many lines of open source code Facebook has contributed, it’d be huge and difficult to compare to other companies that have only been around and contributing for four years or fewer. 

Twitter

Some Twitter open source contributions are so significant, they’ve become even bigger than Twitter. 

Take Bootstrap, which started out as Twitter’s small project to make internal tools look prettier. Since the company put it out into the open source community two and a half years ago, it’s been taken to new heights. Built with Bootstrap and Bootstrap Expo show just some of the thousands of uses other companies have found for this beautifying tool. 

“As of the 3.0 release, we kind of decided to give [Bootstrap] to the community and make it community controlled,” said Chris Aniszczyk, head of open source at Twitter. 

“It’s no longer under our organization, and plenty of its leads aren’t even from Twitter. Projects sometimes become so successful that communities take on a life of their own. Bootstrap fell in that category.”

Aniszczyk said it's fantastic to see a project take off, but the major reason Twitter contributes is in order to “control its future.” That’s why Twitter recently contributed an HTTP request-accelerating program called CocoaSPDY to the open source community. 

“By contributing to the SPDY effort, we can push the whole industry in a direction where people will support SPDY, make the Web faster, and make Twitter faster on all of our clients,” he said. “It’s not just strategic for Twitter, it makes Twitter a better experience for all of our users.”

Every company has a different incentive to contribute to open source. But just like with CocaSPDY, the end result for users is the same. It makes our online experiences better.

Photos by opensourceway, Square, Guillaume Paumier