what chc said plus load times, it would take like 6x longer to load such a web page and as chc points out most of the time you don't really care, now if there was a way to say, take me back to that state or just take me to the top... that might be interesting but that's way too complicated for the average user. (Maybe a FB+ addition)
About the load times, the could be minimized. If you go back to item #235 of your stream, FB could avoid sending you all the 234 preceding items.
Also, yes, i agree in that it could be perceived as a strange behaviour, but to me the current behaviour is also quite strange: if i'm scrolling down the FB stream, triggering a couple of stream sudden enlargements, then go somewhere else on that tab, and then go back to the FB stream, the browser will remember my last scroll position on that page and will try to scroll down to where i was, unfortunately that position does not exists when the page is loaded so it ends up scrolling to a seemingly random point in the middle of the stream (actually it's the bottom of the stream when the page loads, but that triggers a stream-append and then it's not the bottom anymore), not where i was before nor at the start of it (at least this happens in Firefox).
