In terms of status updates (ie, stories which may mention check-ins or photos or similar, but not Facebook Messages), before Facebook's Timeline launch, there were multiple stores of data depending on age. With Timeline, all the different versions of data over all ages were put back together into a single (logical) store. More about that process at:

    https://www.facebook.com/notes/facebook-engineering/building-timeline-scaling-up-to-hold-your-life-story/10150468255628920