The 2002 edition of the Catholic Directory of the US said that there were 75,500 religious sisters, and the 2006 edition said there were 67,773. That's a decline of 7727 in four years.
At the time of Vatican II, there were approximately 180,000 sisters in the US. So in a little over 40 years, the numbers dipped by roughly 112,000. That's a steep decline!
A lot of ink has been spilled over this, but the reality is that nuns are a vanishing breed. The next 20 years should bring a further steep decline, as the average age of most congregations is quite high and elderly members will be dying off in large numbers.
Thankfully there are new orders springing up, but it doesn't seem that they will be able to offset this decline by very much. Many factors have contributed to the problem: Catholics having fewer children, the falling away from faith, other opportunities for young women, religious becoming so much like the laity in many cases that people don't see any reason for religious life, and confusion among religious themselves as to what religious life is all about.
I don't know how it will all turn out, but the Church will weather the storm as it has weathered countless others through the centuries. That doesn't mean that religious life in this country will necessarily climb back to the numbers it once had. I'm guessing it might stabilize somewhere around 20 to 40 thousand, and hopefully begin a small increase again.
For that to happen, though, Catholic family life will have to be renewed. Good Catholic families are a seedbed for vocations.