According to a report by GAO (Government Accountability Office), *two out of three US companies paid no federal income taxes from 1998-2005 (1.3 million corporations with collective sales of $2.5 trillion).
Net Operating Loss Carry forwards (NOLs) a profit in one year may be offset by a loss in the next, ending in zero tax liability.

US Tax Laws- Foreign Tax Credit
Applied where the company has paid taxes elsewhere in the world. The IRS does this to prevent double taxation for corporations (applies to individuals as well). The US corporate rate of 35% could be offset. Exxon's rate was 47%; the tax credit is used to offset part or all of the income tax in the US.

*871,000 US companies paid no federal income taxes from 1998-2005.
The noise is about Exxon Mobil because it is large, but it has a lot of company, no laws violated.

http://www.oil-price.net/en/articles/How-Exxon-paid-zero-tazes-in-2009.php
(The "tazes" should be taxes- error by Google)