Why do links disappear from GWT lists?

[aristotle]

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?

Thank you

[aristotle]

P.S. I should have mentioned that there are some exceptions to the pattern I described above. These are links that have been on the lists for two years or more and appear to be permanent. They include some links from Wikipedia, the Open Directory (dmoz), an arboretum, a few university websites, and a newspaper article. I assume that they stay on the lists indefinitely because they are higher-quality links, even though some (such as Wikipedia) have a nofollow tag. Anyway, these are exceptions to the pattern I described above.

[JS_Harris]

What you're describing is very normal, lets consider where your links are for a moment.

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.

Now lets consider what happens to those pages over a three month period, they fall deeper and deeper into a sites link structure or archive (takes more clicks to get to them) and so the internal link value passed to those pages diminishes... eventually taking them off the GWT radar.

I've heard some people refer to 2nd tier link building campaigns, if you've heard that saying too it means building a link campaign to boost the pages that link to yours. I don't have the patience to do this because I spend my time working on my own content but if you prefer link building to writing articles you can always build a solid foundation for the pages that support yours too. :-)