This is something I have noticed couple of times in the past - it "feels" that old backlinks expire or loose value over time. Mostly for Google, although I can vouch for other search engines, since I've been monitoring this behavior on Google only. I know for sure that certain links do exist, but Google either discounts them or gives them less value with each PageRank update. Those links slowly disappear from back link search results. Am I right or just being too paranoid?
You could be right, if the backlinks are still there and you know your gaining more backlinks continuously then maybe its another factor though. I guess there could be too many other things to consider to narrow it down to backlinks sort of expiring.
I actually had this unintentional experiment - I had a site which was updated now and then, but no new links have been established. After certain time (over a course of few months, actually) a lot of links "disappeared" from backlink list, even though links themselves were there.
That could be possible or maybe they keep it in their archive..that's on my opinion...what about yours..
They very may well expire, but the link would still most likely be worth something, even if only a small amount.
There is no way to get actual value of such links, but I would say their value decrease gradually. The small amount Rimki is mentioning is way too small to even keep the existing PageRank. bomberman, it could be in archives - I don't really care as long as PR-influence is there. It's not and that's a problem. Also, since number of reported backlinks decreases - so is the overall "value" of your site. That's my opinion.
backlinks do not expire but they might get burried or removed by the site where your link was on at one point. for example, if you make a post somewhere (DP, any other forum, or blog), there is a chance that you will get a backlink. what happens if the page where you posted moves to second page or 3rd or even deeper? google may not crawl that deep to get your backlinks updated or the page where your backlink was might not be even indexed by google anymore. so backlinks cannot expire but they can get burried somewhere deep where google cannot index them anymore
Back links are based on the site that they are coming from. If the site loses PR, you will get less juice from them. If the site is closed down, you will lose the link. If the owner decides to remove the link, you will also lose that back link. The upside is that if you have a link on a site that gains PR your juice from that site will help with increasing your PR. This is the reason that it is always suggested to keep building links and doing SEO because there are many factors that are out of your control.
No one contends that you have to keep building links and doing SEO/SEM. Keeping track of your backlinks is just a small portion of it