Some religious groups are much better at retention than others.
Ultra-Orthodox Jews (Hasidic/Haredi) and Old Order Amish have very high retention rates, by some measures >90%.
Mainstream Catholicism has very poor retention rates – there is a quip that ex-Catholic is one of America's largest religions. However, if we look at highly conservative/traditionalist groups like Opus Dei or SSPX, the retention is much better (although I don't have exact figures).
Mormons are definitely showing signs of transitioning from growth to decline, at least in the US. However, given other religious groups I mentioned appear to be avoiding that, it may be more a story of mistakes of the Mormon leadership than anything else.
