Why do links appear then disappear month to month?

[SeanConroy]

What I mean by this is, a link to my site shows up in the August mozscape crawl, then the next month it doesn't, but its still an active link on the website.

Once a site is crawled, shouldn't it always be there from now on?

[AdamThompson]

If you need more complete backlink data for your site, use Google Webmaster Tools and/or Bing Webmaster Tools.

[SeanConroy]

Thanks for the responses. I find it very frustrating that it acts the way it does. Makes it very difficult to explain to the bosses and to give the data any real value. Though I understand the challenges of it all.

Which is why I usually just talk about rankings, if we're beating our competitors in the rankings, then really who cares if they are supposedly beating us in domain authority and such other factors.

[AdamThompson]

Also, SEOmoz doesn't necessarily crawl the same set of URLs each month. Even if index sizes are the same, some URLs get dropped and some added each month.

[MattAntonino]

Rand said on the latest update that

"This index is about 9 billion URLs smaller than our prior September index:  http://www.seomoz.org/blog/september-mozscape-update-is-live, so on average, seeing raw link counts drop ~8.5% wouldn't be surprising."

He also said they would like to get to 150B urls but may get down into the 40B range until some issues are sorted.

http://www.seomoz.org/blog/octobers-mozscape-update

I would say that's the difference you're seeing.