Today is a month since President Obama was re-elected. He has not yet nominated anyone to replace Michael Astrue as Commissioner of Social Security. Astrue's term as Commissioner ends on January 19. Astrue does not continue in office until a successor is confirmed. Once his term ends, he's gone. His replacement is an Acting Commissioner.
Obama has been careful about his appointees. There have been remarkably few bad Obama appointees. However, the pace of Obama's appointments has been slow. At the moment, he's still deciding on his nominees for State and Defense for his next term. There's no telling when he'll announce a nominee for Commissioner of Social Security. After he announces a nomination, it will probably be a few months before the Senate takes action on the nomination -- and that's assuming the nomination is uncontroversial.