Data from Wikipedia for the previous two US WDYTYA series allows us to put the present series in perspective. It has been one of extremes with the Reba McEntire episode having the greatest number of viewers ever, and the previous Blair Underwood episode the fewest ever.
The scatter diagram above shows all US episodes of Who Do You Think You Are? broadcast to far with the total viewers in million on the horizontal axis and Google hits, as of 4 March 2012, for the celebrity name on the vertical axis.
Every episode that had a viewership less than six million has less than 15 million Google hits for the person's name. Generally the more Google hits the higher the number of viewers, but there are other factors, that's a trend which makes common sense.
The next episode features Jerome Bettis who at 1.5 million has the lowest number of Google hits of any celebrity to date. Low ratings are to be expected.
I wonder what the rating would be for WDYTYA Justin Beiber who has 360 million Google hits.