As the name indicates, NARA oversees the American national archives as well as the Presidential Library system, where former presidents deposit many of the papers associated with their terms in office.
The rationale? To use social media to further the organization's mission of providing information to the American public, make people aware of it as a resource and, of course, to get the public to visit its locations.
"By joining Foursquare, the National Archives invites users to learn more about this nation's history via social media. Each tip highlights an interesting place or event related to the National Archives and the Presidential Libraries & Museums."
Alas and alack, unlike "Crunked" and "Slut," you don't seem to get a special NARA badge for your archival peregrinations.
Those unfamiliar with NARA might be surprised to learn how invested in social media it is. Most of the subsidiary archives restrict themselves to Facebook, but the main archive uses Twitter, Flickr and even has a wiki. Most of the libraries use Twitter, YouTube and Facebook. The "Daily Document" is even available as either an iPhone or Android app.
Other sources: ResourceBlog