Stephen Foley: Fine on the Surface but Ballmer has to tell us why that's entertainment

US Outlook: Memo to Steve Ballmer: it's not the device, it's the ecosystem.

When Microsoft's chief executive launched Surface, the company's answer to Apple's iPad, this week, it was notable for being the first piece of hardware of this kind that Microsoft has designed and built in house. Normally, the software giant just ships its Windows operating system to the likes of Dell and Hewlett-Packard and lets them build the kit as they wish.

Apple has shown the benefits of tightly controlling the hardware and the operating software, so as to make sure the consumer experience is silky smooth. Google looks to be going down the same path and could next week reveal a tablet device of its own design, which runs the Google Android operating system and which will be sold directly by the company online. Google is already manufacturing phones, thanks to the purchase of the Motorola handsets business. Facebook, which thinks of itself as a "social operating system", is said to be working on its own mobile phone.

But if controlling the hardware and software together is necessary, it is not sufficient for success. There is some magic fairy dust in the equation, too, and that is the creation of an ecosystem in which developers want to come and produce applications that run on your device, and which enrich and enliven the experience for the users. Put it another way: how much will there be for us to do on the Surface, Mr Ballmer?

A week before Microsoft launched Surface, Apple's Tim Cook was on stage to launch the next version of iOS, the operating system for the iPhone and iPad. He launched that presentation by boasting that 30 billion apps have been downloaded to Apple's mobile devices now, producing $5bn in revenue for developers.

Those numbers demonstrate just how far Microsoft still has to travel, and from a weak third place behind Apple and Google.

What Mr Ballmer has to do is to explain how the new Windows 8 family of operating systems works together across devices from phones, laptops and now Surface, and mount a persuasive case why developers should produce apps that turn a device from a nice-looking piece of kit into a must-buy source of endless entertainment.

The "killer app" for Surface is clearly Microsoft's own Office suite of business tools, like Microsoft Word, which of course does not run on the iPad. There is also integration with the Xbox, another Microsoft creation. But in the new world of consumer devices, one company does not an ecosystem make.