Oh there are plenty of people on - 72 as I write.
But your situation (couple looking for a third who must be female) is one that is very very common - far more common than females looking for this, so don't be surprised if you don't get much response.
Most couples that I know who have had success finding are those that get involved with a local group and meet folks, not as potential dates, but as friends, and then they get to know friend-of-friends, etc.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb