Ben Guthro, a 27-year-old software engineer from Boston, started building a family tree on Geni.com after he saw a post on Digg. His tree now has 250 members, some of whom he has never met before, such as his mother's cousin who lives in California. "It spreads fast," he says.

More-established genealogy sites are also adding Geni-like elements. For instance, The Generations Networks — founded as MyFamily.com Inc. nearly a decade ago and now operating eight sites including Ancestry.com — has built its $150 million-a-year businesses primarily by selling subscriptions to passionate family historians for access to archives that track family lineage, such as census records, draft cards and death certificates. The Provo-based company is offering more free, ad-supported features, allowing family members to build trees by attaching ancestors' names, photos and stories. Since adding these features last July, Ancestry.com users have created more than 1 million family trees, with 150 million names, 400,000 photos and 10 million family documents from the site's thousands of record collections.

Geni.com works like this: Users enter their first and last name, e-mail address and gender. A box then pops onto the screen, pink for women and blue for men, where members can add a picture, then fill personal information into a profile. Members then create their family tree by entering the names and e-mail addresses of their family members. Those relatives receive an e-mail from Geni asking them to join the network and help build the tree by inviting more relatives and creating profiles for ancestors. As family trees start to overlap, Geni.com plans to link similar branches together to create one giant family tree where members can track their ancestry and see their distant relations living throughout the world. Right now, Geni doesn't offer access to any data bases or other research sources.

Members also are using Geni to stay in touch with living family members through sharing pictures and sending messages. Geni hosts more than 1 million profiles, with average users adding 10 people to their trees. Ana Greene, a 28-year-old from Arlington, Va., is planning a wedding and intends to post pictures from the event on the site.