WIRED.com reports that Facebook employs Mac Mini farms in its data centres.

Facebook’s iOS app is so complex — designed to run on so many mobile devices across the world, and built with code written by so many Facebook engineers — that the company needs an enormous network of machines to test it. “We run hundreds of thousands of tests every couple of hours,” says Christian Legnitto, the ex-Apple man who oversees the Facebook engineering team responsible for the final release of the company’s mobile apps. “When you’re shipping [an app] to hundreds of millions of users, those one-in-a-million bugs can happen hundreds of times.”

But because Apple insists that all Apple software run on Apple machines, Facebook can’t test its iOS app on the Linux servers that drive the rest of its empire — or on the popular cloud services offered by tech giants like Amazon or Microsoft. So, says Legnitto, the company operates “racks and racks” of Mac Minis that run Apple’s Mac OS X operating system.

Last week, I reported on Google using 40,000 Macs.