Well, I think the idea was that if a sorority has a lot of nice, classy women that behave nicely and don't act like big crazy hookers or whatever, then guys will want to marry them. And that getting engaged was a sign that fraternities on campus or guys on campus generally thought the group was full of nice women that they wanted to date, which = popularity.

I've also seen it used more strictly, as in "percentage of seniors engaged to members of this 'top' fraternity."

Don't get me wrong, I actually strongly, STRONGLY disagree with this idea. But this was how the idea was put forth by those I saw advocating it.