Correction: you _deactivated_ your account. Facebook keeps your data.
Facebook doesn't make it clear about what types of data they do and don't keep beyond saying they disassociate you from the information, but retain some for 'technical' reasons. I can't really imagine what those would be.
[1]http://www.facebook.com/help/?faq=125338004213029#What-is-th...?
Probably the thing about databases getting fragmented, so they delete large swaths of data and optimize all at once, rather than piecemeal when the users delete it. It's well known that when you "delete" a photo from Facebook, if you save the direct link to it first, you can still access it (go ahead, try it). But the one time I tried it, the photo did finally disappear after about a week (although I have seen some people claim that they could access deleted content via direct links for up to 6 months).