I initially dismissed this, but looking at the last 4 months on one of my sites, they are sending remarkably consistent traffic, and the site in question has gone up the serps for most of my main target phrases - which would imply the traffic is being throttled somewhat:
(excuse the formatting)
Jan
68 different referring search engines Pages Percent Hits Percent
Google 485167 76.6 % 487166 76 %
Feb
60 different referring search engines Pages Percent Hits Percent
Google 466708 75.8 % 468233 75.5 %
March
67 different referring search engines Pages Percent Hits Percent
Google 521946 75.7 % 523921 75.4 %
April
62 different referring search engines Pages Percent Hits Percent
Google 516085 74.9 % 517908 74.8 %
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