Are there any numbers of criminals (un)caught by wiretapping the internet?

All over the news you see of the NSA or other countries wiretapping the internet. And all numbers available are about how and how much data the national agencies are collecting. But if it comes to the point to 'is it really necessary?' then everybody is uses the argument of the feeling of security. So are there any hard numbers available of how many criminals are caught by wiretaps? Did this number increase because of the wiretaps? and how many criminals are (probably) uncaught by not having a wiretap?