I'd say it depends on the demographics...
For the 15 to 25 year old group, it may be as high as 80%. Young ones are computer literate and are used to the Internet being pretty much a part of everyday life. Also, this group is more inquisitive and rebellious than other groups. They're more prone to question authority.
For the next group up, say 25 to 40, it probably drops to around 50 percent, maybe less. Many are now married with kids, holding down a job. They're just going through the motions. If they got this far without really questioning the religion, they may not be motivated to look unless some major event were to happen, like a family member needing blood, or a seperation/divorce or something similar. Others are still searching for themselves or trying to figure out why things don't feel right in the organization.
The next group, 40 to 60, is even less...probably 25%...maybe. This age group in the real world has discovered the wonders of the Internet, but anyone at this age who has been in the "truth" for most, if not all of their life, is going to be afraid of the Internet. Again, it would take a major event to cause some of this age group to question things. Just having the Internet is not enough of a temptation to look at things that could shake their faith.
By the time you get to the 65+ group, it will be slim pickings to find anyone in the JWs who uses the Internet, let alone looks at anything remotely "apostate". All the people in this age group that I know that are JWs, none have the Internet and none want it. They've gone most of their lives not needing it, why would they need it now?
My mother (in the 65+ age group ) used the Internet in her job, but never used it for personal reasons. She never had the curiousity to see what was out there. She bought the WTS warnings without question. When she retired, the family offered to buy her a computer so she could get the Internet and email and she said she didn't want it. My mother-in-law (also in the 65+ group) has dial-up Internet just so she can have email to keep up with grandchildren in different states. She doesn't care about the World Wide Web. She constantly scolds my wife for spending time on the Internet after all that the WTS has said about how dangerous it is.