I updated the graph with another interesting curve. Bitcoin first hit the news in 2011 with a couple of slashdot and wired posts. So if we take from there to today, and fit an exponential, we get a doubling every 150 days or so.  So with this hindsight it's clear to see that the June'11 bubble was truly a bubble. Growth since then has been intermittent but constant over the long term. I'm not one to speculate, but the graph suggests bitcoins bought today will be worth 4 times as much in 12 months.

http://imgur.com/Hs6zMoi

@timo: I wouldn't say exponential growth is necessarily bad, but it is certainly always unsustainable. Perhaps that's what the bitcoin critics refer to.