Elsewhere in the thread, I commented on how due to having non-globally routable blocks, as per their ISP service agreement, some sites are not reachable from other sites. I remember thinking at the time that some of the click counts were off, but there was no (obvious) sign of fraud, so this could be attributed to clicks that AV got, but the user was not redirected to the target site because it wasn't reachable. This has some implications for an IPv6 world, or even an IPv4 world where routing tables overflow sufficiently often to produce serious reachability problems. Implicit in the online advertising models is that "all sites are mutually reachable, all the time".
BTW, I joined the NANOG main list, and I may join the IRTF RRG (Routing Research Group). They are discussing a way to shrink the routing tables by tunneling non-core traffic through the Internet core, among other things.