Apparently the number of nuns (and monks) in the Roman Catholic Church has been on a steady and strong decline since Vatican II, although the overall membership of the RCC has continued to steadily increase. During the reign of John Paul II, the ranks of the nuns declined by about 25%, and a new report indicates that from 2005 to 2006 the ranks thinned by 10%. (source)
The reason? Death.
Old nuns are dying or and many other nuns are simply abandoning their vows. There are new recruits, but the new recruits are not keeping pace with the exiting folks.
The current breakdown according to the article is:
1.1 Billion Catholics
0.09% are in monastic orders (total: 945,210)
753,400 are women (about 0.13% of all Catholic women)
191,810 are men (about 0.04% of all Catholic men)
The Reformation churches have consistently held that monastic vows are unlawful. While we are not rejoicing because of the death of nuns and monks, we are glad to see the ranks of the monastic orders dwindling. It would be a delight if, by God's grace, the entire monastic system were abolished. This is an example of change for the better brought about by Vatican II, though perhaps not by intent.