It looks from this like several million will be the correct figure.
I heard that only about 3 million bitcoin addresses are used. I would say that the number of addresses > number of users.
Myself, I have used dozens of addresses, and don't consider to be a heavy user of addresses.
http://bitcoinrichlist.com/charts/bitcoin-distribution-by-address
Total 11,462,007
10mils really don't matter so ......
The addresses above 0.001 threshold could almost directly be mapped to holders 1:1. But I know that holders use several addresses so that has to be accounted for.
What are the 10 million dust addresses anyway? Nobody buys with $0.10
I know I'm gonna regret for saying this but..
People who heard about bitcoin , downloaded bitcoin-qt, tried to earn some satoshi with faucets and gave up.