An Hour Alone With Rarity by Phil Argus
Link: http://www.fimfiction.net/story/19755/An-Hour-Alone-with-Rarity

"One very lucky guy takes part in a seemingly harmless Facebook challenge which inadvertently wins him an exclusive visit from arguably the most beautiful pony in Ponyville, who appears to have 'other' things on her mind. Contains human-on-pony, memorable Rarity quotes, and Haddaway. Based somewhat on true events."