FXCM does not trade against you. We make our profit from the trading volume via a mark-up on the spread. Click here to see a video on how our No Dealing Desk execution model works. Below is an excerpt from page 9 of our Client Agreement that discusses Order Execution. I encourage you to read the entire document and let me know if you have any further questions.
FXCM provides all forex execution through straight through processing, or no dealing desk execution model. In this model FXCM passes on to its clients the prices available by FXCM's liquidity providers with a fixed mark-up per currency pair, according to the order's size, order type and available rates. FXCM is not a market maker in any of the currency pairs it offers.
Jason