MBlume probably would have been more correct to say, "At least in the US, far more people self-identify as Mormons because they were raised by LDS parents than because they had a long talk with two guys in white shirts."

About two-thirds of official members are converts, but these numbers don't reflect active members. Because retention rates for new converts are around 25-50% after one year, it's plausible that any given active member is more likely to have been born in the church. The LDS-released statistics don't paint a good picture of who actually attends, but without other data, this is hard to answer one way or the other.