Mark is a brilliant coder. He's not a brilliant visionary nor inventor. Let's not forget that social networking sites had existed long before FB so he wasn't breaking new ground. His product has certainly been more successful, but that's not to say it's been executed flawlessly (or anywhere near there). My point, however, was simply that introducing a FB branded phone would be nothing more than a gimmick. Maybe FB would put together a familiar variant to all the other phones out there or maybe they would put out some cutting edge hardware...none of that matters. The technical challenges aren't in processor speed, memory, size, weight...the challenges are in defining a completely new or largely different way to interact with the phone. Otherwise, they'd essentially be doing the same thing as every other handset and OS developer with the exception that perhaps FB would be even more tightly integrated into the OS.
Again, FB is just one of many services people use. They're not like Google or Apple or Microsoft; these companies offer many different services that people use so it's not a silly idea for them to "roll their own."
To clarify my argument here, I'm saying that a mobile device (phone, tablet, whatever) developed by FB wouldn't be a great business move in the long term. I don't know if they'll actually try to do something like that, just that I don't think it'll work for them.
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