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Some sites have their logs (or sometimes just the results of their analytics software) visible on the web. If a spider crawls these reports/logs, these supposedly referring websites show up as links. There are also some apps out there that display the most recent referers to a site's webpage on that page, so I suppose it's a way to get some human traffic as well.
Sometimes half the traffic in a day is apparently from this same porn site. I use AWStats (I have Webalizer available but AWStats seems to give more consistent results). I traced some of the IP's and notified the ISP's but no idea if they bothered to do anything about it. :-(
The other 20% of the referrer links are to non-existent pages on my sites. These non-existent pages follow a pattern, usually something like: /webmaster.php /register.php /cgi-bin/register.php... etc.
I was supsious that they were trying to find a membership login/registration page on my site and hack in to it to perhaps promote their site? It that possible? or likely?