A software error on Nasdaq OMX Group Inc's U.S. exchange delayed the social networking company's market debut by 30 minutes last Friday. Many client orders were delayed, leading to significant losses to some investors and traders as the stock price dropped.

The exchange operator is facing lawsuits from investors and threats of legal action from brokers.

Claims from market makers look to overwhelm the $13 million fund the exchange aims to set up to deal with potential claims. Nasdaq's liability is limited to $3 million in certain cases, but electronic trader Knight Capital Group, for one, has said it will fight to be fully compensated.

According to sources with knowledge of the situation:

* UBS AG had trading losses of around $30 million stemming from Facebook's IPO;

* Knight Capital Group saw losses of between $30 and $35 million;

* Citadel Securities booked losses of $30 million; and

* Citigroup Inc's Automated Trading Desk notched losses of about $20 million, according to a source.

A market maker is a firm that stands ready to buy and sell a particular stock on a regular and continuous basis at a publicly quoted price.

Other firms are expected to report additional losses from Facebook's IPO, including a $1 million loss by E*Trade Financial, potentially adding millions of dollars in cumulative claims, sources said.

Regulators, including the U.S. Securities and Exchange Commission, the Financial Industry Regulatory Authority and Massachusetts Secretary of the Commonwealth William Galvin are now looking into how the IPO was handled. The U.S. Senate Banking Committee is also reviewing the matter.

(Reporting By John McCrank; Writing by David Randall; Editing by Walden Siew and Kenneth Barry)

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