Fatto generale: 81609 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 81609 is an unremarkable number.
Trivia: 81609 is an unremarkable number.
Data storica: June 9th is the day in 1900 that Birsa Munda, an important figure in the Indian independence movement, dies in British prison under mysterious circumstances.