Answer:
   
   Certainly, assimilation has been a problem for Judaism. Reform Judaism
   is especially subject to that problem because many assimilated
   Americans who want to identify as Jews, choose a Reform synagogue, not
   because they are Reform Jews, but because the Reform temple is the
   least judgmental of their assimilated practices (or in some cases,
   non-practices).
   
   Many of the children of these families do not remain Jewish, but in
   fact assimilation was already a generation old.
   
   To actually be a 3rd or 4th generation Reform Jew would require that
   one's ancestors, living in the 1800s, be Reform Jews. The major growth
   of Reform Judaism has been in the 20th century. So while it is true
   there are few 3rd or 4th generation Reform Jews numerically, they are
   not a large number primarily due to the heavy immegration in the early
   1900s. The majority of today's Reform Jews have ancestors who were O
   or C, but primarily because there was a limited pool of R on whom to
   draw. Yet there are 3rd, 4th, and even 5th generation Reform Jews
   (just look at the FAQ Maintainer's daughter).
   
   There is a different question, however: For those that were Reform in
   the 1800s, how many of their children are still Jewish, and still
   Reform, today. Anecdotal evidence suggests that number is large.
   
   Note that Reform has invented and reinvented itself (and will
   undoubtedly continue to do so) in response to a changing world. By the
   1930s, it became clear that "classical Reform" wasn't speaking to the
   majority of RJs. As a result, the evolutionary changes that began in
   the 20s were formalized with the "[5]Columbus Platform"
   ([6]http://ccarnet.org/platforms/columbus.html) of the 30's that led
   to Traditional (or Modern) Reform.