As Facebook’s much-anticipated public stock offering approached, federal regulators wanted to know more about the revenue it gets from mobile devices, its $1 billion deal to buy Instagram and the control CEO Mark Zuckerberg has over the company. Documents filed Friday show the back-and-forth the social-networking company had with the Securities and Exchange Commission on a variety of issues. As all companies do, Facebook Inc. amended its regulatory documents several times ahead of its initial public offering of stock. But the communications leading to those changes weren’t public until now.