From the White House blog:
Recently, we have seen a few stories questioning how the Presidential Records Act (PRA) intersects with Americans’ use of modern social media, like Facebook and Twitter, to communicate with the White House. Before I address some specific issues raised in these stories, let me give you a little background. . .
. . . These new types of communications from individuals to the White House, even though they take a different form, are governed by the PRA. Working with NARA, we’ve concluded that comments and messages the White House receives on its official pages are presidential records. That means the PRA requires us, by law, to preserve them.