DW is in bed, but I think I can answer most of this for you, because I had some of the same questions and thoughts.
First of all, I wouldn't make the assumption that BM had more FB friends than SM. BM and SM were a couple of years apart in high school and it just happens that DW is closer in age, and therefore was friends with BM. She said BM was extremely inactive on FB, to the point that she (and others) wondered why he even bothered having a FB page at all. She said that the only reason she wasn't friends with SM is because she just never thought about it - I think had BM been more active on FB, it likely would have triggered that thought for her years ago.
As far as the friends/trust part of this - I think that has a lot to do with being from such a small town. The average graduating class in Alexandria is maybe around 100. There's one high school, one middle school, and only a couple of elementary schools. My wife has friends that she's known since second grade, male and female. I went to a high school with 10 times as many students, and just can't relate. So I think this is normal - I've gone with my wife to her high school reunions, and there's a closeness that just isn't there in larger communities/schools. I'd also say this about that closeness - I think there's accountability that's built in there as well - in such a small community - if there was anything inappropriate going on between married classmates, you better believe it would get out and people would call them out on it. All that to say, I wouldn't read anything into BM having FB friends that weren't also friends of SM.
I can't speak to content on BMs FB page (probably violates TOS anyway), other than he just wasn't active on it.
Hope that helps.....