I often imagine a future where our society has collapsed, then rediscovered by archeologists. These archeologists have to re-piece our history using what is left of the internet. They run into a problem, Facebook is the only thing left of the internet which turns out isn't a bad thing. The collective experiences of our civilization is captured in this information and is easily able to be recovered. The reason for this is that no account was ever deleted and never expired, thus creating a catalog of our experiences since inception.
Facebook now records our experiences on a "Timeline" so we can rewind a persons history and look back into their past, see their happy moments and sad moments.
When we die, Facebook does not close the account but rather modifies it (once notified by next of kin). The modified profile becomes a tribute to the individual's life cementing their online experience into a memorial. The new memorialized page becomes a place where friends can come and post their note on the deceased members wall and recount their experiences through their life.
At the end of the day, unless every person who owns a Facebook account deactivates it before they die, It will be up to the next of kin to either shut it down or memorialize it for their friends. In other words becoming a mass e-grave for lost profiles.