Facebook man Joel Pobar calls it "a holy-fuck comparison." The massive collection of software code needed to create that Facebook page inside your web browser, he says, has now expanded to the point where it's about the same size as the code that underpins the Windows operating system.

He won't say how many lines of code we're talking about here, and in all likelihood, Windows is still the larger creation. But Pobar is in a better position to make this comparison than most software developers. He oversees the Facebook engineering team that makes sure all that programming code can be quickly converted into the stuff you see when you visit Facebook.com, and in a past life, he worked on various programming tools at Microsoft, the maker of Windows.

Whatever the exact numbers, Pobar's holy-fuck comparison highlights just how far the web has come over the past decade. Facebook is a single web service – a single application, if you will – but it looks an awful lot like an operating system. By one measure, it's approaching the complexity of an OS, and in many ways, it behaves like an operating system. As Mark Zuckerberg is so fond of saying, Facebook is a "platform," much like Windows, where you can run all sorts of other applications, from email and IM tools to games and photo apps. Nowadays, many people spend far more time on Facebook than they do on Windows.

Jason Evans, one of the Facebook engineers who work under Pobar, won't divulge the size of Facebook's codebase either, but he says that public statements have revealed the number in the past. At one point, he indicates, the Facebook codebase was pegged at 10 million lines, and later something close to 20 million. The implication is that the codebase has since grown beyond this number.

In January of 2011, in a post to question-and-answer site Quora, Facebook engineer Evan Priestly said that Facebook spanned 9.2 million lines of code – a figure that didn't include various services used to support the main Facebook application. Jason Evans says that this post was spot on, but then he points out that it happened two years ago – an eternity in the life of Facebook – and he confirms that the figure only applies to a portion of the site as we know it.

>'It's harder to maintain code that that's big. It's harder for any one person to know about everything. The bigger the ship, the harder it is to turn.' Sam Schillace

Though Microsoft declines to say how many lines of code underpin Windows, a company once said in a post to, well, Facebook that the code for Windows XP spanned 45 million lines. That seems like a stretch for Facebook, but even if the social networking site is in the 15 to 20 million range, it's line with the Linux operating system. The Linux kernel – the core of the popular open source operating system – now spans about 15 million lines.

Linux developer and pundit Jonathan Corbet questions whether Facebook's claims would hold up to scrutiny. "The Linux kernel is the result of 21 years of work by several thousand people," he says. "The idea that Facebook has pulled together a similar thing, with a similar level of effort, is not entirely credible to me." And in the end, these comparisons are hard to make. It's difficult to know where to draw the line between a core codebase and the various tools that support an operating system or a massive web service like Facebook. But Facebook's Pobar is simply saying that Facebook is now in the same ballpark as Windows, and the company's enormous codebase is in part the product of Facebook's very different approach to software development.

Sam Schillace – the vice president of engineering at Box.com, who also helped build Google Docs, the search giant's online word processor – says he believes the claim that Facebook is now in the same ballpark as Windows, though he adds a caveat. "Facebook is optimized for innovation. They're not necessarily optimized for elegance. That's their Hacker Way. They just want to get things done. Probably, that code base could be half the size it is if it was more carefully designed."

The larger point here – the point Pobar is trying to make – is that Facebook now faces an excruciatingly difficult task. Juggling that much code isn't easy – particularly when you have to deliver it over the web at speed. If you assume that Facebook now spans 30 million lines of code, Schillace says, that's the equivalent of about 300 paperback books filed with programming language. "It's harder to maintain code that that's big. It's harder for any one person to know about everything," he says. "The bigger the ship, the harder it is to turn."

This is one of the reasons why Facebook's engineering staff looks a lot like what you'd find at places like Microsoft or Oracle or VMware – i.e. the world's leading software companies, outfits that build software not just for themselves but the rest of the world. Drew Paroski, another engineer who works under Probar, also came from Microsoft. And third, Keith Adams, came from VMware.

Even if Facebook is as large as Microsoft Windows, Box's Sam Schillace questions whether Facebook's code is as sophisticated as the stuff that underpins Windows. But at the very least, it's moving in that direction. In more ways than one, the web is the new operating system.