Helpful,
I am not aware of any study that applies our standards for defining a halachic Jew to the members of Reform congregations. Anything we say here is anecdote or speculation. I was mainly judging by the dues-paying membership and especially by the fact that the number of Reform synagogues in America has increased substantially over the past 30 years.
The 2001 National Jewish Population survey showed a decline of 13% in the number of persons identifying as Reform Jews since a similar survey thirty years earlier, and the same 13% decline in self-identified Orthodox Jews. Conservative declined 50% during the same period. I think that much of the Orthodox decline may be explained by the total destruction of the once huge Jewish community in most of the Bronx, much of which was at least nominally Orthodox.