The Techmeme Leaderboard is a valuable tool for tracking the influencers of the tech world. However, over the last few months an interesting trend has emerged: A-list bloggers are dropping off.

Take a look at the Techmeme Leaderboard from last October and compare it with the full results from yesterday's Leaderboard. Robert Scoble's Scobleizer, which used to be #55 on the leaderboard, has dropped to #70. Others have dropped off completely, including Nick Carr's Rough Type, which used to hold the #35 position, Andy Beal's Marketing Pilgrim, which ranked 86, and Doc Searls, who ranked 97 back in October.

What's going on here? Certainly, seasonal factors are coming into play. Nick Carr, for instance, announced in June that that he was putting Rough Type "on ice" over the summer months. News events, product releases, and conferences may impact the makeup of the board as well, depending on the expertise and opinions of the sources, and how they are picked up by other sources.

But one thing that I find particularly interesting is that the old-school A-listers aren't being displaced by the many talented B-to-Z listers out there. Rather, the Leaderboard is increasingly populated by mainstream publishers, tech blog networks, and corporate blogs and PR sites. There are a few personal blogs on the July 15 Leaderboard that weren't present last October, such as those written by Jason Calcanis and Dare Obasanjo. But I see these as exceptions. (Also, Calcanis is a member of the old A-list establishment, who is likely on the Leaderboard now because he received a spike of inbound links after announcing his blogging "retirement" last week).

Another interesting fact about the Leaderboard is the cutoff point for inclusion has risen. Techmeme assigns "presence" scores to the hundreds of blogs that have featured headlines. The methodology listed on the site has this definition for presence:

"A source's presence is the probability that a random Techmeme headline at a random time over the past month was published by that source."

At first I thought some sort of random sampling method was used, but Techmeme founder Gave Rivera told me this is not so. Rather, the Leaderboard calculates the entire census of headlines in a 30-day period, weights them for the amount of time spent on Techmeme (measured every five minutes), and then calculates the 100 most-used sources. The many links in the Techmeme "Discussion" blocks are not counted -- only articles or posts whose headlines are shown on the Techmeme front page.

In October, the lowest presence value was 0.17% for the #100 position, the Syndney Morning Herald. Yesterday, the lowest presence value was 0.20% for 9 to 5 Mac. In other words, even if a blogger's presence was enough to make the top 100 in October, there's no guarantee that the same presence value would get them on the leaderboard yesterday. There were 10 such sources that scored below .20% in October, including Doc Searls.

When I asked Rivera about the changing presence threshold, he responded with this:

"Nothing profound to say about the change. For all I know, it may have been both higher and lower in the interim. Things move around a lot."

However, when I took a look at the presence levels at one-month intervals starting on October 1, 2007, there seemed to be the start of a pattern: Lower thresholds (between 0.15% and 0.18%) persisted from October 1 to May 1. Since June 1, 0.19% and 0.20% presence levels have been the norm.

If the high presence thresholds persist, individual bloggers who are unaffiliated with any network or news organization are less likely to make the Leaderboard, regardless of whether they are A-listers or Z-listers. Rivera had an interesting observation about the way Techmeme and the