Firstly, hacker won't be able to withdraw your balance to his own wallet. Because freebitco will send a confirmation message to your email if the hacker use different address to withdraw. So they try the multiply btc game with the amount. Secondly, the hacker maybe got your IP address, email, password from a phishing website and gained the access of your account by setting up the same IP on his device. Cause you would receive a authorization email if anyone else tried to login from a new IP.

This kind of thing also happened before with other user. There was a case where hacker increased the account balance of a user by playing the multiply game. The hacker turned 0.018~ btc into 0.45~ btc and didn't lost it (See the post).