Late Friday, Swiss bank UBS confirmed that it, too, had lost money trading Facebook’s shares in the days after the company’s IPO. The bank wouldn’t say how much. But the Wall Street Journal and CNBC put the loss at $350 million, making it the largest Facebook loser so far by far. Originally, Wall Street’s Facebook trading loss was estimated to be around $100 million. But in the past week or so, some were putting it at double that. Add in UBS’s previous unknown losses and Wall Street’s Facebook hit could be more than half a billion.

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UBS is a market maker, and like other brokers was trying to help its clients get Facebook’s (FB) stock. But it’s hard to believe that UBS (UBS) could lose this much money just buying and selling the stock for others. The company said it tried to sell the Facebook shares it had bought at $35, but ended up exiting some of its shares at below $30.

Facebook’s shares opened at $42. So if you assume that UBS bought at the open – it wasn’t an underwriter on the deal so no preferred pricing – and sold at an average $30, that would give the bank a per share loss of $12. Compare that to a total loss of $350 million, and what you get is that UBS bought just over 29 million shares of Facebook on the day of its IPO. Meaning UBS bet $1.2 billion that Facebook’s IPO would pop. And that’s if it got out at an average $30. If UBS instead sold at an average of $35, more likely, then the firm’s bet on the stock could have been as big as $2.1 billion.

Some of that buying could have been for clients who the firm assumed would want to buy, but never materialized. But more likely a good chunk of it was by some trader at the firm, or many, who tried to goose their book on the assumption that Facebook’s IPO would shoot up.

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UBS says that it suffered its losses because of glitches, and that it is considering suing the Nasdaq. But Nasdaq says it has gone over every Facebook trade in detail, and that it has only found $40 million in losses that can be attributed to problems at the exchange. Of course, it’s likely that Nasdaq is low-balling. But it’s unlikely that its figure is off by a factor of ten. And even if you assume the actual amount attributable to Nasdaq is some where in the middle, say $200 million, it still means that Wall Street firms placed huge bets that Facebook’s IPO would soar.

For me, the interesting thing here is what this says about the Volcker rule, which is supposed to stop firms from making the type of huge risky gamble that UBS seems to have made on Facebook, and how much gambling is still going on on Wall Street in general. A number of Wall Street firms say they have wound down their so-called proprietary trading desks, but given the large losses in Facebook, that doesn’t seem to be the case. What’s more, UBS is saying it sustained its loss in its market making unit, which is a business that is allowed under Volcker. The rule is not in effect yet. But it’s clear that even after it is, Wall Street will continue to be able to hide a good deal of its risky trading from regulators.