Looking around, I've found this Oct 22, 2012 article at Tech Crunch which says what the largest web-based service is:
Ancestry.com is the world’s largest online family history resource ... for its 2 million-plus subscribers.
Ancestry.com's Form 10-K Annual Report Mar 8, 2011 tells us about the top selling genealogy program:
Family Tree Maker is the leading family history desktop software on
the market, with over 1.7 million units distributed since 2004.
An Oct 24, 2012 ABC report by Alan Farnham about Permira's $1.6 billion purchase of Ancestry.com says:
Hard numbers are difficult to come by, but hobby experts believe that
genealogy ranks second only to gardening as American's favorite
As for how much bigger it might grow in future, Sullivan
says: "Think about the last big family Thanksgiving dinner you
attended. If there were 18 people, chances are that at least one of
them is interested in family history. If you apply that same metric to
our existing market, we think there are 20 million potential customers
for our services. Today, we're serving on 2 million. We think there's
If (a big if) as Tim Sullivan, President and CEO of Ancestry.com said: 1/18 was the ratio of people interested in family history worldwide, then there would be about 400 million people interested in family history.