They went from the estimated 5 billion birds at the beginning of the 19th century to the last known one dying in 1914. They were killed on mass by commercialized hunting for meat. One of the methods for hunting them was to take live pigeon and sew its eyes shut. It would be tied to a wooden stool. The fluttering of the bird would attract other pigeons to see what was wrong. The hunters would then drop a net on them and crush their heads between fingers and thumbs. This incidentally may be the origin of the term stool pigeon.
nods to the http://thememorypalace.us, http://en.wikipedia.org/wiki/Passenger_Pigeon, and http://www.petermaas.nl/extinct/speciesinfo/passengerpigeon.htm for the info.