In the 1910s, propagandists would send push-surveys that included a question "(who else/what other organisation) do you know who should get this survey?" in order to map out socio-political graphs and plan their campaigns.
HREFs sound like much less manual work.
Edit: > ...able to find similarities even among websites that Marginalia doesn't index
Yeah, I joined LinkedIn specifically because even without my input it was way too good at figuring out my actual social graph. With my input, it no longer is :)