What you're suggesting is only relevant if we assume that everything posted on facebook is true. What if Grandpa was a little story teller himself and everything that came out of his mouth - or on his timeline - was already bullshit? The past that we uncover is ultimately - and unwittingly - a mediated artifact simply by the fact that it is not experienced in the present.
That's so, but how many people already assume that everything they read on the internet is true? Or from historical sources? Not only that, but remember that timeliness are not only populated by self-posts; how many times have you been tagged in a photo taken by a friend, for example? Anyway, the whole subject is only valid if users do in fact, use these social media sites for their entire lives and if they continue to exist far enough into the future that a situation like the one proposed by the image. Given the speed with which the internet changes and that younger users are eschewing Facebook because their parents are on it, suggests to me that in as little as five years from now, social media will be radically different and may be structured very differently from our experience of it at the moment. For one, what if all the current issues regarding internet surveillance and tailored advertising, including the permanent retention of information รก la Facebook, are considered to be unacceptable? I doubt that will happen, but I do think that the internet will continue to be pretty volatile for quite a while.