I think that this is a British thing - we tend not to be "up front" with religion. In the interests of "research" , I've checked the entries of a few JWs and most of them do not mention being a JW. However , in comparison , I checked a few prominent "apostate" JWs who are now evangelical or born again Christians (including some well known faces from this board ) and these don't mention their religious affiliation either....
I guess that if there was an American Friends Reunited equivalent , people would be more inclined to put their affiliation on their profile - but it's just not the "done thing" on this side of the pond.