I heard about this incident on btc-e trollbox and came to have a look.

On the homepage it now has this message which is a good sign.

Please do not deposit any kind of coins.

Please cancel all orders and request a withdrawal.

CoinMarket.io will be shut down when the problems are solved.

With regards to exchanges and I have been involved in a couple, (even investigated doing my own) security is a big concern. There is 2 coins hackers and scammers are concerned with
and that is bitcoins and litecoins. Don't listen to the noise regarding alt coins. Every exchange that has been "hacked" have given back alt coins. This is includes freshmarket.io, openex etc and everytime
the hacker is demanding ransom in order to have btc/ltc returned which does not make any sense at all.

With the Poloniex hack where 12.3 btc was stolen it only affected the btc that was in the system at that time. IE Lets says I had btc on poloniex before the hack then I would be affected, however btc deposited after the hack was not affected.

What remains a mystery is the plan for the bitcoins that were deposited after the hack like others are starting to query. Coinmarket has made it clear that they are involved in negotiations for btc
that was deposited during and before the exploit.

So possible reasons why btc deposited after the hack haven't been returned?

1. They plan on using all btc to lessen the damage to the rest of the people that had btc stolen. So everyone gets a smaller amount back even if you deposit btc now.
2. They are going to return all btc back once the negotiations for making up the lost btc has been concluded.
3. You will never get your btc back.

Does anyone know how much bitcoins have been stolen?

This is a really interesting scenario playing out here and definitely will come back to read updates. Lets hope this story ends well for everyone and that the bitcoins are returned.