For this series, I am going to use the Quantcast statistics because they are easily understandable, and free. There are other websites that measure traffic, and their traffic statistics may be different from Quantcast. To simplify things, I will show only the number of Visits - not Page Views or other statistics available from Quantcast.
The first website for this analysis is www.Ancestry.com. Here is the daily traffic estimate for the period 12/06/2008 to 11/30/09:
The average monthly number of U.S. People visiting Ancestry.com over the past five months was 5.5 million people. The graph above shows an average number of Daily U.S. Visits to be about 1.9 million over the last five months. Note that this data does not include persons visiting from other countries.
Quantcast has statistics for about three years - here is the chart for Ancestry.com since 02/05/2007: