The total amount of bitcoin in circulation decreased by almost 9 in 2017 due to “burn” addresses. Burn addresses, like the Genesis Block, are deadlier than “zomby” addresses, because there is no chance of coming back once BTC is sent to it. There are almost 400 known bitcoin burn addresses, i.e. valid addresses with no private key, to which, for a variety of reasons, people have sent 2,759.42507135 over the years. In 2017, 9 of these addresses received 8.97140133 BTC.
http://bitcoinwhoswho.com/blog/2017/12/30/8-97-bitcoins-burned-in-2017/I'm curious what their motives are. What really makes people send their coins there? Though I don't agree that the total amount of bitcoins in circulation decreased that much or that little in the departing year. Apart from the wallets the private keys from which are lost for good, we should also include the addresses with dust which are no longer usable due to hefty transaction fees.
And it seems that the total amount of coins in these wallets by far exceeds 9 bitcoins that got burned.