Because there are not a lot of useful metrics that allow it to be accurately calculated, it's hard to say.
On top of it, for those that decide to conciously "fade", having access to forums such as this one allow them to do so in a more discreet manner. Also, anecdotes that we all read seem to indicate that there may not be necessarily a greater percentage that are leaving, but there may be an ever growing number of "nominal JWs", who still attend meetings, report some service time (whether it's accurate or not), and have serious doubts about doctrine, but are not willing to turn their personal lives upside down over the issue.
One metric that can be accurately measured, is not whether more are leaving, but if fewer are responding to the JW message and joining. I don't mean in terms of net increase-- there is still low single digit point growth every year-- I mean of all the people JWs preach to, the percentage who actually respond continues to decrease. Somebody's got a graphic out there that tracks how many hours of preaching it takes to get 1 convert over the last few decades. That number has been steadily increasing and shows no signs of slowing down.
Is it because JWs are less effective teachers?
Is it because more and more JWs over-report (lie) on their monthly service reports?
Is it because fewer people are interested in religion?
Is it because people who are interested do some research on JWs on the internet and decide to stay away?
There are no clear answers, but if any or all of the above are true, it will only cast a longer and longer shadow over the Watchtower Society as time goes by.