I have come across adult tube sites that have backlinks from domains like xxxaa1.com, xxxbn2.com, etc. They are mostly registered from China, and they seem hacked link directories. They show up on Majestic but when opening those sites the respsective backlink usually no longer exists or the site has already been taken down. Their formats are too structured (xxxaa, xxxab, etc) for there not to be any trick behind it. Are these paid backlink networks? Who operates these sites and why would tube owners want backlinks from them?