aaron barry berry
niall teague
KPMG
Effective February 26, 2009, Apple's independent registered public accounting firm is Ernst & Young LLP.
Ernst and Young is the auditor of Facebook Inc.
PWC
From Exxon's official webpage states: "Independent, registered auditors PricewaterhouseCoopers LLP..." So, PWC is the auditor.
deloitte
KPMG
The firm Deloitte & Touché LLP is the independent auditor for Twitter. They handle all of the financial statements for this company.
Deloitte & Touche LLP
PricewaterhouseCoopers, LLC
The population of Apple Inc. is 2,011.
The main reason an auditor cannot invest in companies they audit is of course that there is a conflict of interest. An simple example of this would be an auditor, auditing Apple Inc's accounts, the auditor would have prior knowledge on the companies profit, which would not be public knowledge until the results are made public. Based on Apple's performance an auditor who have information that could be advantage regarding as another example the possible up or down side of the companies stock price. There are numbers other examples such as an auditor conducting due diligence on a merger and acquisition. But the main reason is a conflict of interest
The population of Apple Inc. is 60,400.