Even if all turns out to be some kind of big misunderstanding in the end: a service holding bitcoins in the tens or even hundreds of thousands of bitcoins for thousands of users simply cannot be left unattended for a period of three days. If nothing else, the owners of mybitcoin are unprofessional and careless, which invalidates their service for further use.
The only reasonable thing for everyone to do, if they come back is: get your bitcoins the hell outta there!
+1. They've already been down for a length of time exceeding anything reasonable (made much worse by the complete lack of communication). Even if it's not a scam and MyBitcoin comes up in the next hour, they're still completely incompetent, and not deserving of
anyone's repeat patronage.