It's a special promotion that a few sites are participating in. To get the
three days free you have to go to the site and click on a promotional link that
will take you to
ancestry.com Sites that participate get some kind of points
from ancestry, as I understand it.
The "virus" that was going around that was supposed to be an ancestry link
turned out to be a hoax. It was just one of ancestry's promotional links that
started out with "service.bfast.com" (as all of ancestry's promotion links
do).