While most sources I’ve seen date link farms to Google’s arrival on the scene, Wikipedia puts their origins a bit earlier. They were supposedly used in 1999 by SEOs to take advantage of Inktomi’s algorithm, which depended on link popularity. Back in the day, Inktomi’s primary index included only 100 million listings. Pages that didn’t have a lot of links regularly fell out of the primary index; link farms were supposed to help stabilize listings for web sites that had few natural inbound links.
When Google came on the scene, the link farmers had to adjust their techniques some, because it didn’t weigh votes the same way. Not every vote was equal. Still, link farming continued to prosper for a while, and eventually became automated. Link-finding software emerged, designed to hunt down potential link partners, send template-based emails, and build directory-like web pages containing the links.
It should surprise no one that what one computer program can create, another can detect. Link farms leave distinct patterns. Search engine algorithms can be tuned to these patterns. Once a link farm is detected, it’s easy enough to remove it from the index. And more often than not, that’s exactly what search engines do nowadays.
There’s a very good reason for this. Take another look at the end of the last section, and how big a link farm can become. You’re talking about 100,000 sites, each with hundreds of pages made up of nothing but links. Usually, with a link farm, there is no real information about any of the sites; there may be a one sentence description, but it could just as easily include only the links without a description. The links may be in no particular order and show no sign that any of them are related to the site hosting them in any way.
If you ran a search engine, would you want that kind of thing cluttering up your index? Of course not. Your job is to serve your visitors with the most relevant results possible. Link farms don’t help that cause at all. They’re no longer necessary for helping web sites stay in primary indexes; indeed, they haven’t been needed for that purpose for years. Even as far back as 2002, Kimberly Krause Berg, writing for Search Engine Guide, pointed out that link farms were “illogical, especially if your website was trying to generate sales leads or sell products or services. The appearance of link farm pages made some web sites appear unprofessional.”
Link Trading 
/ 19 
