Fatto generale: 1684 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1684 is the number of multigraphs with 6 vertices and 9 edges.
Trivia: 1684 is an unremarkable number.
Data storica: August 10th is the day in 1969 that a day after murdering Sharon Tate and four others, members of Charles Manson's cult kill Leno and Rosemary LaBianca.