RPW's, et al's paper was made public today, and demonstrates several practical attacks on Hidden Services. http://www.ieee-security.org/TC/SP2013/papers/4977a080.pdf
"pg 80: Until now there have been no statistics about the number of hidden services..."
There are some... at least one current crawler project counts around 1100. I've not got far enough into the paper to see what their 'until now' means.