The great big Canadian family: One in 2,200 chance that a complete stranger is your cousin
This is mostly marketing hype based on an interpretation of data from 4,800 Canadian customers in the AncestryDNA database that statistically 1 in 2,200 Canadians are cousins.
Read down to the footnotes in the news release and cousins is defined as the total number of 4th cousin relationships and closer. However that's qualified if you delve further into an AncestryDNA match for 4th cousin you read it's "Possible range: 4th - 6th cousins."
Within that range there will be many more people who aren't exact 4th cousins. In January 2010 this blog had a chart showing the number of relatives for the THREE family, where all ancestors came from a family with three children in all branches of the tree. For that artificial case you would have 2,592 4th cousins, more than four times that number of 4th cousins once removed and nearly six times that number of fifth cousins.
To delve more deeply take a look at Blaine Bettinger's statistics showing the range found in the amount of shared DNA at a given relationship. Unfortunately it doesn't go out to 4th cousin but notice there are cases of no shared DNA at the 3rd cousin level.
Nice infographic. I'm dubious about the statistics but have a query in to Ancestry hoping they'll be willing to clarify.