One of the most knowledgeable ex-Mormons I know is Richard Packham. He is not the kind of person who makes extravagant statements about Mormonism (true, it's usually not necessary to embellish it, right?)  He has these very interesting insights about how many Mormon missionaries leave the LDS church after serving full-time missions. Here are Packham's words (reprinted with his permission):

I first heard that 40% [of LDS missionaries eventually leave Mormonism] . . . from a post on an Internet discussion
group dealing with Mormonism a few years ago by someone who had a relative working at the MTC [Missionary Training Center.] This relative told the poster that the staff at the MTC had been urged to do nothing to damage the new
missionaries' testimonies, because 40% of them will end up out of the church.

At the time, since it was not verifiable, I viewed it as interesting, but not reliable.

Only a few weeks later I was attending a conference in SLC [Salt Lake City] and was
introduced to a young man who had just recently finished his mission,
and who had left the church. I mentioned this post, and the 40% figure,
and asked him what he thought about it. He said that when he was in the
MTC the missionaries had been told the same thing: if you are not
careful, 40% of you will eventually leave the church. That seemed to
confirm what the original poster had said.

I think most members are unaware of how many people leave the church.
Even Mormon sociologist Armand Mauss estimates that among new converts
in the US and Canada, 50% are no longer active one year after baptism,
and that in other countries the figure is 75%. That seems to be
confirmed by looking at census figures in countries where the religion
of the citizens is tabulated. The number of Mormons found by the census
in Chile in 2002, for example, was 103,735. But according to the church,
they had 520,202 members listed on their records. That would indicate
that 75% of their members of record do not identify themselves as
Mormon. And of course a member who had resigned or been excommunicated
would not appear on the church records as a member, but would raise the
number of exmormons.

In Mexico the situation is similar. The 2000 census showed 205,229
Mormons in Mexico, but the church had almost 850,000 members on its records.

RMs [Returned Missionaries] are a very prominent presence on all of the ex-mormon discussion
boards, although there is no way of counting them.

FWIW [For what it's worth] my very devout grandson served a mission in Germany, married in
the temple, and surprised us all a couple of years ago by leaving the
church (I had nothing to do with his apostasy – I was as surprised as
his parents were). And it's not just the missionaries who “didn't have
their heart in the work.” My grandson, I'm sure, was a hard-working,
faithful missionary. And John Williams, the author of the book I
reviewed [see below] also was evidently a hard-working, faithful missionary. He,
too, has left the church, although one would not guess that from his book.

I would suspect, too, that missionaries who were BIC [Born in the Church or Born in the Covenant] and from strong
Mormon backgrounds (such as your family) will have fewer apostates among
their missionaries than among those missionaries who are relatively
recent converts, whose families are not strong in the church, or who
served a mission primarily due to pressure from the Mormon society and
not from personal conviction.

[Book that Packham reviewed:]
> Title: Heaven Up Here
> Author: John K. Williams
> Publisher: Amazon Digital Services
> Genre: Autobiography
> Year Published: 2011
> Number of Pages: 341, (367 Kb)
> Binding: e-Book
> ISBN10: N/A
> ISBN13: N/A
> ASIN: B005WYQ7SI
> Price: $9.99

For more information, see The Mormon Mirage 3rd Edition:  A Former Member Looks at the Mormon Church Today (Zondervan, 2009). Also available as an audiobook and as an expanded-text E-book for Nook, Kindle and other reading devices.

Also: Latter-Day Cipher by Latayne C Scott.