I've created a few charts that look at the number of priests and members of religious orders over the last several decades. To avoid it looking like spaghetti, I only used the top dozen orders based on number of priests*. I also split the top 4 from the other 8 - there is very little overlap between the two and it makes the charts much clearer.
The charts are posted here. (.pdf format)
On the number of priests, data mostly goes back to 1966 through 2012. The overall trend appears downward (-30%), but there are several exceptions.
The Salesians (-1%), Conventual Franciscans (+5%), and Discalced Carmelites (+13%) have basically been stable over the entire time period. The Divine Word Missionaries actually had a substantial (38%) increase.
The number of members data goes from 1950 to 2012, generally. Again, the overall trend is downward (-28%), but there are also exceptions. The most obvious difference from the previous charts is that thanks to the extra 15 years of data, it is clear that there was a substantial bump up across the board leading into the mid-1960s which then fell away in later years.
The exceptions to the trend are: Conventual Franciscans (+26%), Discalced Carmelites (+28%), and Divine Word Missionaries (+45%). Salesians deserve a mention again have only a small loss (-5%).
If I look at the overall trend from the high point of each order through the most current data, the trend looks much worse: down 39% overall.
From this perspective, even our previous exceptions have a slight downward tilt: Divine Word Missionaries (-2%), Conventual Franciscans (-10%), and Discalced Carmelites (-6%).
The Divine Word Missionaries deserve a special mention as the only order of the top dozen whose highest number of members was after the 1960s (it was in 2009).
(* Out of curiosity I checked the top dozen orders based on number of members - it was the same dozen, but in a slightly different order.)
Note that only Male Religious Orders were included because that happens to be the data I have readily available. As time permits, I'll try to do similar charts for Female Religious Orders.
Note II: I updated the charts and the stats above with several additional years of data, including 2012 (from Annuario Pontificio 2013). I've also added a second chart covering just the order Legionaries of Christ here (.pdf format).