The personal data of some 100 million people who have used Quora, a popular question and answer website, has been compromised, the company disclosed. "We recently discovered that some user data was compromised as a result of unauthorized access to one of our systems by a malicious third party," wrote Quora CEO Adam D'Angelo in an online post. "We are working rapidly to investigate the situation further and take the appropriate steps to prevent such incidents in the future," he added. The intrusion was discovered Friday.