I've seen this question a lot, but never seen anyone's clear, concise answer or hypothesis to why Google doesn't consider a lot of backlinks in webmaster tools or link:domain.com. Is there a set of factors that Google uses before designating something a backlink? One factor I would have to think would apply would be: 1. The linking site must appear in the Google index. Please add yours!
It's pretty simple, really. The links you see in the 'link' command, or in google's webmaster tools are the backlinks that actually counted towards that domains PR rating. That's why you never see 'no follow' backlinks listed in either of those places. Keep in mind that sometimes google 'resets' the backlinks that count towards a domains PR when if a domain expires. Essentially, google doesn't want someone who picks up the expired domain to reap the rewards of the footwork that the old dwmain owner worked for. That's why sometimes you see a domain with a PR4, but with no backlinks visable on google.
That is true, but it's more than that. They intentionally don't show all of the backlinks pointing to a site, in an effort to thwart spammers. If they showed ALL of the backlinks for each search, you could start running queries, capture the data, and really start to reverse engineer how things work.
I've seen matt allude to backlinks shown by the link command or in webmaster tools as being a 'subset' of the number of backlinks a site has, but i think he means it's a subset of all of the backlinks, with all of the backlinks being ones that don't count a=towards PR. After all, backlinks that count towards a domain's PR value is a 'subset' of all of the domains backlinks. I'm a pretty firm believer that the links shown by the link command and in webmaster tools are all of the links that count towards the PR value. Or more accurately, it will be all the backlinks a domain's PR value is the next time the PR is calculated for that domain. I do quite a bit of that already, but it's still difficult to know how google weighs each of the baclinks. For example, a backlink from a page with 1000 outbound links on it is given much less weight than a backlink with only 10. Not all Google backlinks are created equal. If a site has 100 google backlinks, it could be that only 1 of them has any significant weight.
False. The VAST majority of backlinks are crap. Google knows it, so they don't count them when they calculate PR. That's why, for the most part, you only see QUALITY backlinks from the google link command, or in google webmaster central.
Link farms, paid directories, and backlinks acquired before a domain expired are all reasons why google wouldn't use them to calculate PR.
You want to see more in Google? put a space after the colon in the link check and Google will show more results link: www.yoursite.com
At first I thought this was rubbish, but after looking over some quick statistics I think you might be onto something, as most of my numbers are actually pretty close. I think that the "devaluing" of directories and paid links might be throwing numbers off, so if you're not actively linkbuilding, your number in Webmaster Tools might continue to decline.
Unfortunately, that's not the same thing. if you use a space after the link:, then it is just searching the web for two different key phrases, "link:" and "www.yoursite.com". You'll get almost the exact same result as if you just search for "www.yoursite.com".