What timeline is is a way of strolling through all of your Facebook activity since you first enrolled (and indeed, by adding retrospectively, from before that). To be sure, it offers no new information but just a presentation of that information that serves a function: you can reminisce. I spent sometime looking back on what I was doing in 2007. While it can be viewed by all of your friends (and indeed beyond if you so choose), it is about as private a product as Facebook have ever released. Who else is going to trip down your memory lane but yourself? (Well, others might occasionally but not as often as you would).
Strategically, this is as close to a perfect example of leveraging lock-in as can be envisaged. If the fact that all of the people you know were on Facebook wasn’t enough, now your past is there too and if you want to access it and build on it, you need to stay on Facebook. Moreover, Facebook already have the technology that makes it easy to record activity which is what had made it successful in the first place. Now it gives you another reason to be active: so that you can look back and see what you were doing. In other words, activity is not just social, it is a potential life investment. Facebook will not only be the largest news organisation ever but now may well become the largest archive ever.
Clearly, this brings lots of things into sharp relief. But, in particular, it is officially the case that your life starts at 13 years old. What a shame then, as you look back, you have to start with your teenage thoughts rather than what you might have thought before that.