In economics, the Gini coefficient is the standard measure 
  of how inequitable a society is. This is tricky to 
  determine for Bitcoin, as it's not quiet a "society" in 
  the Gini sense, one person may have multiple addresses and 
  many addresses have been used only once or a few times. 
  (The commonly-cited figure of 0.88 is based on one small 
  exchange in 2011.) However, a Citigroup analysis from 
  early 2014 notes: "47 individuals hold about 30 percent, 
  another 900 a further 20 percent, the next 10,000 about 
  25% and another million about 20%"; and distribution 
  "looks much like the distribution of wealth in North Korea 
  and makes China's and even the US' wealth distribution 
  look like that of a workers' paradise

  Dorit Ron and Adi Shamir found in a 2012 study that only 
  22% of then-existing Bitcoins were in circulation at all, 
  there were a total of 75 active users or businesses with 
  any kind of volume, one (unidentified) user owned a 
  quarter of all Bitcoins in existence, and one large owner 
  was trying to hide their pile by moving it around in 
  thousands of smaller transactions. (Shamir is one of the 
  most renowned cryptographers in the world and the "S" in 
  "RSA encryption")"

[1] "Attack of the 50 Foot Blockchain" via https://news.ycombinator.com/user?id=davidgerard