However, if you want to count the actual numbers of conversions happening right now, the statistics are messy: apparently just about every religious group (including the group of non-religious) claims they are the "fastest growing", all with some empirical justification. I found this Wikipedia article highly amusing in that context.

Note that if at the beginning of the year A had one billion adult adherents and B had one hundred, and since then 160 of the former have converted to B and 60 of the latter have converted to A, my heuristic would still point towards B being wronger than A even though B has doubled in size and A has stayed pretty much the same. (And anyway, I was thinking more of memeplexes who have existed for at least a couple of generations -- with new ones it would be much more noisy.)