This is normal for a deposit to an exchange: the funds are often swept into another wallet, or used for fulfilling another customer's withdraw.

Just like if you deposit a $10 note at the bank, your balance remains $10, even though that same physical bank note may end up in another customer's hands.

I can't state categorically that your funds are safe at Cryptopia, just that you should not be alarmed by your deposit address showing a balance (on the blockchain) of zero.