I went to the Cambridge, MA LessWrong/Rationality "MegaMeetup" and it was almost exclusively students

Weird; that's not my memory of it or my perception of the group. The meetup was at Harvard, which meant we had a couple more students than usual, but I think 80%+ of the local people at the meetup were out of school.

At the meetup yesterday night, which I remember better, there were about 15 of us and I think only one student (late 30s statistics grad student).

Is there a Boston EA community substantially disjoint from this LW/Rationality group that you're talking about?

There's a lot of overlap, but it's a separate group. Looking over the rsvps at our most recent dinner I count 8 people who also go to lesswrong, and 15 who don't. On the same list i count three students.

are there many historical examples of movements that experience rapid growth on college campuses but then were able to grow strongly elsewhere?

The history of movements is something I'd like to know more about, but haven't really looked into much. (One thing I found frustrating when I did is that there's a huge amount of survivorship bias.)

Facebook did this, though it's not a movement.

I expect a lot of the rosy pictures people have of translating money or other fungibles into EA converts will not stand up to scrutiny

I agree, and am similarly pessimistic. But $100k is still a lot of money, and we don't yet have that much experience trying to figure out how to spend it.

I do not hear many people describe it as "I had heard about these ideas a few times, but it was only when Bob [who was supported by EA funding] took the time to sit and talk with me for a few hours that I was convinced."

There are very few Bobs who are supported by EA funding, but I can think of several people who switched to EA after lots of talking with existing EAs. Right now we have relatively little personal outreach and relatively more digital/idea-based outreach, so we should expect to meet more people who were receptive to the arguments when they heard them remotely.

did the church reduce its emphasis on having children so that parents would have more free money to give to the church?

I'm not sure the church was strategic or flexible enough to do this, and even then I doubt kids were anywhere near as expensive as then. Specifically, I think the age at which a kid went from net-consumer to net-producer was something like 9 compared to today's 22. (But I'm not very informed on this.)

historians ... we could use a lot more data and systematic analysis.

Yes!