Therocktrading.com an Justcoin.com claim to have about 7000 registered users each.
The Rock's 24h Volume is at about 60 BTCs and Justcoin is at about 90 BTCs/24h (http://bitcoinwatch.com/).
Combining the number of users and the 24h Volume and dividing it by 2 we arrive at an average of ~0,01 BTC traded in the last 24h per member.
Comparing this to the BTCs traded in the last 24h at btcchina.com results in a number of ~9 million users for BTC-China.
Given that BTC-China makes up about one third of the entire volume of all markets (http://bitcoincharts.com/charts/volumepie/) we arrive at an overall estimate of 27 million people trading bitcoins at exchanges.