There is a big thread on this on reddit for those who want more info:

https://www.reddit.com/r/BitcoinMarkets/comments/4vtv1m/bitfinex_down_due_to_bitcoin_security_breach/

P.S. The following thread from 11 months ago says they used defunct code from a website that had already been hacked:

https://www.reddit.com/r/Bitcoin/comments/3igv0r/bitfinex_pretty_sure_we_are_dealing_with_amateurs/

Bitfinex was released within some weeks of the Bitcoinica source code leak.
The codebase was copied from a defunct, thrice robbed company.
That code is not capable of keeping the database consistent at peak loads.
That code is not scalable. That code was not, and is not correct.
Not correct according to specification. (Which doesn't exist.)