Well, Im not sure any statistical 'facts' have emerged in this thread, but I could offer up an anecdote. Of the 10 or so people I know personally that have Linux installed on computers/laptops in their home, 7 of them use it exactly as a desktop. ie. their primary or only OS for everyday computing. Ive been using it that way for 15 years now. I *am* a hobbiest, but my hobby computers are legacy unix machines, like DEC, HP, Apollo, Sun, SGI, etc.
Now, none of these people have ever purchased the Linux they run, so they are hard to count. Their ages span a range from 40 something *cough* to gradeschool, and they all live in midwestern US states.