"Facebook would be better if Facebook used files" is a bit of a strawman. [Edit: No, it isn't. Crikey, failing at English: what is the word for an argument premised on a proposition that is contrary to fact? It can't just be "wrong", that's what 3rd graders say -- needs more Latin.]

Do you know a single application that abstracts away as much data as Facebook does as files? Anything on my computer nearly that complex, like an email inbox, a photo collection, or my iTunes stuff exists as an opaque blob with a proprietary reader. (In the case of my inbox, it is an opaque blob with widespread inport/export capability into opaque blobs of different colors.)

It has a file on disk like MySQL does: OK, technically accurate, but I'm not capable of actually doing any of my usual file activities with that file.

That is aside from the "whose data are we talking about, anyhow" conundrum. Probably 80% of my presence on Facebook "belongs" to other people. Tagged in a photo taken by my mother of my brother's wedding: think quick, is that in my export or not? (For that matter, is my brother in my export? I'd kind of like to import him into my new social network. You can do that, right? Oh and Farmville.)