I did the FTDNA test which is mainly American, and has a massive membership, as you might expect. My Scottish name is fairly well spread throughout the States, and I was approached by lots of people trying to establish a link to the UK.
One person had a database on Ancestry.com with 24,000 names, and now that my family tree is on her database, I don't get as many contacts, thank heaven. I got interesting info from her research, of course.
The Americans are nuts about family hidtory, being such a young and somewhat rootless people. Each of the several DNA groups has a website, some of which have overwhelming amounts of information, far more than I could ever want to use.