1. What was Facebook's accounting effective tax rate for 2018? What items caused the company's accounting effective tax rate to differ from the "hypothetical" tax rate of 21 percent? What was the company's cash effective tax rate for 2018? What factors cause a company's cash tax rate to differ from its accounting effective tax rate? You can access Facebook's Form 10-K for 2018 at the company's website or the SEC EDGAR website.