It is a difficult question to answer, since you do not need to register to use the script. Upon installation or first run, the script makes a request to my webserver for a file (either an image or a script). I keep track of those requests.
This is the information I have for 2014, for installations from Greasy Fork, OpenUserJS, and userscripts.org combined:
1/1/2014-3/31/2014: estimated 7,642 new and upgrade installations
4/1/2014-6/30/2014: estimated 3,400 new and upgrade installations
Many sites have links to this script on userscripts.org. Since those broke in early May, that might explain the significant drop in installations.