https://i.imgur.com/Yw81i.pngThe historical data for mtgox has been archived somewhere but here is a chart of the volume data ...
and the 70-80% market share was active during the death spiral of MTGox itself ... after "withdrawls" were being held was when they saw the most drastic changes in their daily volume and epople started switching over immediately... but some didn't as other exchanges had lower volume , liquidity , and prices...
Additional source on the manipulation http://bitcoin.stamen.com/ 