Members of sites including Facebook, Bebo, MySpace, Hi5 and Orkut can download the free Family Tree app to their online profiles, set up family profiles and connect with other relatives who’re members of the same site.

With around 22 percent of Americans using social networking Web sites (says the Pew Research Center), piggybacking off them is a particularly effective way to market a product.

More than 3.5 million people currently use Family Tree. According to Familybuilder spokesperson Myles Weissleder, Internet traffic information provider Comscore rated Familybuilder’s site the 4th-most-trafficked online genealogy service in March. (Other top sites were The Generations Network sites, MyHeritage, Genes Reunited and FamilySearch.)

“As we enter into our second year, we’ll be offering more tools and services to make researching family history fun,” says Familybuilder CEO Ilya Nikolayev.

That includes a way to link up with family members across social networks, so a Facebook fan, say, could add a Bebo-using cousin to her family profile.