freejung - 5:47 pm on Sep 2, 2010 (gmt 0)
I think we're talking about two, possibly three different possibilities.
The "keyword pie" idea doesn't have to do with traffic directly, but rather with the number or type of keywords you are allowed to rank for. The idea, if I understand it, is that Google grades your site as a whole and decides, OK, you' can rank for ten high-popularity keywords, 500 medium popularity keywords, and 2000 long tail keywords, and that's it. Even if you add new content, you're not getting any more of the pie. Or maybe, OK, you can rank for keywords involving fuzzy widgets, but not keywords involving smooth widgets, regardless of how you optimize.
Am I understanding the concept correctly?
Traffic throttling is a slightly different concept, it's been discussed in a lot of threads. Basically, Google might decide, OK, you're going to get ~5000 referrals per day, and that's it, no more, no less. So if your ranking for one keyword goes up, your ranking for others will have to be reduced to compensate. By slight adjustments in ranking over thousands of keywords, it wouldn't be too hard to do this (at least in my niche, where the traffic numbers for each keyword are very predictable).
Then there's the "damping factor" idea, which basically says, you're not going to move very much in the SERPs. Wherever you rank now, you're going to rank similarly tomorrow no matter what you do. A sort of molasses, that makes movement in the SERPs slower and more difficult. This would have a similar effect of making your rankings and traffic very stable, but it wouldn't limit you to any specific traffic number, just make it harder to increase traffic in general.
Whatever it is, I'm pretty sure there's something new going on in the last few months (roughly). It's much harder than it was before to rank new content for new keywords, and also to improve rankings of established content. If it was just me, I might put it down to increased competition in my niche, but it sounds like other people are observing the same thing. What it really reminds me most of is the old "sandbox" effect, only applied on a page-by-page or keyword-by-keyword basis rather than to whole domains.