FXCM writes in the risk warning that a negative equity can happen but does not mean you have to pay. You can loose all of your money but no more. It happened to me too.

"Although the margin call feature is designed to close positions when account equity falls below the margin requirements, there may be instances when liquidity does not exist at the exact margin call rate. As a result, account equity can fall below margin requirements at the time orders are filled, even to the point where account equity becomes negative. This is especially true during market gaps or volatile periods. FXCM will not hold traders responsible for deficit balances in this scenario, but clients should be cognizant that all funds on deposit in an account are subject to loss. FXCM also recommends that traders use Stop orders to limit downside risk in lieu of using a margin call as a final stop."

http://www.fxcm.com/legal/trading-execution-risks/