That will eventually lead to decentralised applications, but in the mean time will simply lay the groundwork for it whilst people continue with the centralised model.
There will always be a need for centralised systems, the world is just not going to simply change their entire toolset and paradigm overnight. We must make centralised apps use pretty much the same technologies as decentralised ones first, then coax developers over.
There is no reason (and you provide no argument) that using the same code on the client/server results in decentralization. Furthermore, we have the fact that it already exists and still did not lead to decentralization.
You'd get drafted into storing images for some subset of the intersection of your social graph and several others, and the rest of your images would be served by someone(s) with whom your social graph intersects.
Presumably invitations would be via e-mail, or, if you wanted to be even more decentralized, QR code.
As far as data volume, I've got to imagine the data usage per user would be significantly less than BitTorrent.
The desirable aspect of this system is obvious - the only people who see your data are people who've seen a QR code from someone in your social graph - your FaceTorrent profile will never, ever be indexed by Google.
What if Google started offering, say, free Google Coupons (assuming they roll their own groupon) in exchange for befriending them on FaceTorrent? Not all of my friends and relatives value privacy as much as I do.