I'm curious as to what % of possible addresses have been used? 
considering the amount of bitcoin addresses that can be generated, I'd say we have used around 0.000000000001%
FYI, today I've done a full dump of all bitcoin addresses ever used at least once. I've reached the number of 401,407,598 unique addresses for all block files from blk00000 to blk01275. The % is still really small. Smaller than 10^-40 %, at least.