Parker Conrad has resigned as CEO of Zenefits, following a number of regulatory compliance failures at the richly valued human resources startup he co-founded, according to an email sent to employees on Monday. David Sacks, the chief operating officer, who formerly was an executive at PayPal and Yammer, is taking over as CEO. Zenefits also named Joshua Stein, a former federal prosecutor who is a vice president of legal affairs at the company, as its chief compliance officer. Sacks attributed Conrad’s departure to compliance failures by the startup.