was the entire site really behind tor? if that is the case, i wish you guys the best of luck to find him, otherwise if it wasn't just get the federal bureau of investigation or some other entity that cover cyber crime. they can force the hosting company to reveal the identity of the person/s. an incident this large is almost certain to get caught. and when he/she is caught, you might be lucky and get all your coins back assuming the logs were still in tact/obtainable, or at least be ordered to pay back the BC.
It's going to be hard to get any authorities to take this seriously unless the coins are actually moved. Right now the evidence of wrong-doing is limited to a website being down, something which I doubt is an offence in any First World nation.
Given that Mybitcoin went for maximum anonymity when creating the LLC and registering the domain, I'd be surprised if the hosting company has any details about the actual owner. It's most likely that was arranged through their company agents as well, which means law enforcement would hit a dead end unless they're willing to devote a huge amount of resources to sifting through the hosting company's records to try to determine where the unknown owner logged in from.