The thing about permanently backed up demographic info though: it changes. My age, for example, changes every year. My location changes every few years. My interests change. Hell, even gender changes.
All of those things are fairly trivial for FB to track. Age (obvious), Location (trackable through Places, geolocation, etc. through that thing in your pocket), Gender (presumably you would post a status update about something that big?), Employment (status, amassing colleagues as 'friends'?).
It seems like they've hit on a constantly self-updating demographic info machine...
I was referring to your mention of "permanently backed up", i.e., stale demographic information, whose value decreases predictably with time. While FB is in business, you're spot on.