For many older users, myself included, it is doubtful that we will go back to the time before Facebook (for me, late 2004) to fill in the gaps on my Timeline. From talking to other people my age since the announcement, it seems more likely that people of our generation will sanitize stuff that resurfaces rather than adding more content to the stream. But for younger users, they don’t need to fill in any gaps — their Timeline is already more or less complete.
Nice opinion piece that captures something I was taking about at our Korrio exec team meeting this morning. For those of us that joined Facebook in our post-college years the idea of going back to "fill in the blanks" may not be that appealing. For younger users, including those who have grown up with the Internet, Facebook has been their constant and consistent companion. They've switched phones and schools, but Facebook has always been there collecting their thoughts, photos, accomplishments and events.