Apple has always been a hardware and software company with a very specific philosophy about user experience being everything. The digital music revolution gave Steve Jobs the opening to create the iPod that basically resurrected the company. That ply has been extended into phones and mobile computing.
Google is (from a revenue perspective) a search and advertising company. Their business model is predicated on how much you use the internet. So what Google is concerned with is everything between you and the internet. They've done Chrome, Android, etc to commoditize every link in that chain. Everything they do is about you using the internet easier, cheaper, in more places or more often.
Microsoft is about one thing and one thing only: selling Windows and Office licenses to enterprise and OEM customers. Nothing else matters. Look at the Kin: they bought Danger (with its once successful Sidekick) but then spent the better part of two years rewriting its OS as some Frankenstein version of Windows mobile. It arrived way too late and was dead before the first unit was sold (in large part due to the enormous data costs that Verizon saddled it with as they'd lost patience with the delay). Windows Phone 7 has the same goals.
Facebook is all about capturing and cultivating your relationships. They are a silo. They want to own that data because that data is their business and their entire value. Integration with everything else is about allowing other people to give Facebook more data.
What sense does a mobile platform make in that context? Absolutely none that I can see. How does it let them capture and control more data? Apple has had huge success with the iPhone because they have the industrial design, UI experience, supply chain management (which shouldn't be underestimated), iTunes (as much as many--including me--hate the application it is a near-monopoly in digital content) and retail channel.
For Facebook to succeed in the mobile space they would need most if not all of these things. It's a huge risk for relatively little gain and a small payoff. I can only thing that by leveraging a mobile platform of their own they could essentially lock out other platforms. The problem is that Facebook is usable with any Web browser.
I can't imagine Facebook doing any more than controlling the Facebook experience on all mobile platforms, an area they're still sadly lacking in (iPad app anyone?).
Basically this makes as much sense as Twitter making a mobile phone.