As if you were writing a letter about a terrible fail that just happened to Failblog.org.
Similar to saying "Dear Penthouse Forum..." after something very sexy had just occurred, like sleeping with two unbelievable girls at one time.
Man 1: "Dude, this six hour trip to see Ozzfest was totally wicked and fucked up cause it took so long, but we are finally here bro! Hey, get out the tickets, the ticket taker is right over there."
Man 2: *checks pockets* "Dear FAIL Blog..."
Man 1: "YOU SON OF A BITCH!"