Genealogy and family social networking website Geni.com, exceeded 1 million unique visitors for the first time in August and continues to grow rapidly.

Since Geni spends nothing on marketing, its growth is entirely organic and driven by the virality of its family tree application, which asks users to invite a relative when they add him or her to the tree. That relative can then invite other relatives.

The largest tree on Geni now contains profiles of over 600,000 people and was built by over 40,000 users. No other genealogy site can boast of so many users simultaneously participating in a single tree. As a free, easy-to-use web application, Geni is able to mobilize the efforts of anyone interested in their family, in addition to genealogy enthusiasts.

Recently, Geni released a tree-merging feature, which enables two trees to merge when a common relative is invited to both. This process can be repeated many times over, leading to the discovery of new family relationships and strengthening Geni’s network effects compared to other genealogy sites.

David Kaleita, a Geni user, stated, “As recently as six weeks ago, I was connected to approximately 3,000 profiles (2/3 of which were manually entered by me and the rest manually entered by other family members) before merging with anyone. Now, several tree merges later, the stats report that I am now connected to 263,288 profiles! More than half of that growth came within the last two days.”

By default, user profiles can be seen only by those in their own blood tree or in directly-connected inlaw trees. Inlaws of inlaws are limited to seeing search results only.

Geni is like a time capsule for your family that you can open anytime. With some family participation in Geni, descendants will no longer have to wonder what their ancestors were really like. It is backed by venture capital firms Founders Fund and Charles River Ventures.