someone send me a message that I won and I received 0.41 BTC on that site.

Deposit before withdrawing
It mostly happens in gambling section but I have seen people coming up with currency exchange service and try this method.
The trick is very easy. You will see an ANN or service announcement of a new website out of nowhere. You will be given X amount of free BTC to try their website. You will spend those fee BTC and play games in their site. Instantly you will find yourself very lucky. You will make Y times of the X and once you will request a withdrawal they will ask you to deposit Z amount BTC. Once you will contact the support they will give this explanation that the primary deposit is required to verify your account. I have seen people to be trapped and lose their money. Do not send any single satoshi to anyone who will come to you with this method. Stay away, you have been warned.

Example victims or offers:
Stay away from Fake Coinbase giveaway ! (accusation, send btc first)
be aware. exmofit.com scam website (accusation, this time it was a fake exchange)
In progress

Don't be deceived. A site being on .com doesn't prove it to be legit or not. After all, BTT (this site we're on) is on .org, and not .com yet it's as legit as it can be. The .com thing is an old trick of scammers. Flee, bro. BTW, always use coinmarketcap or coingecko to access any exchange you want to make use of to ascertain level of authenticity.