Google Webmaster Tools shows regularly-updated lists of external backlinks that googlebots find for my sites. I've been watchingthese lists for about two years and have noticed a recurring pattern:
Step 1. A new-found backlink appears on a list for the first time, having been recently discovered by a googlebot. Sometimes this is a link that I created, and sometimes it was created by someone else. Mostly they are low-quality links from blogs, free directories, forums, and social networking sites, but a few are better-quality links from independent websites.
Step 2. After a new link appears, it remains on the list for a period of about 1-3 months, usually through a number of updates.
Step 3. But then the link abruptly disappears from the list, never to be seen again.
My basic question is this: When a link disappears from a list, does this mean that Google no longer gives any value to it?