Shelfari chief executive and co-founder Josh Hug said in the company blog that the acquisition would enable the small company to create "innovative new tools."
"With more resources and Amazon’s expertise in building a platform where people come to share ideas, there are a lot of new opportunities in the future that will benefit each of you," Hug said in Monday's posting.
Shelfari would continue to operate normally with no immediate changes to its services, Hug said.
Shelfari is a nearly 2-year-old social networking Web site in which users build virtual bookshelves of the book titles they've owned or read. Users can rate, review, and discuss their books.
Shelfari recently launched applications for Facebook and MySpace members to access Shelfari services through the larger social networks.
Amazon.com in February 2007 invested $1 million in Shelfari, which was launched in October 2006. At the time of the investment, Hug said Shelfari remained an independent company and planned to use the new capital to accelerate its growth.
Books are a major product line for Amazon.com. The online retailer made a big push in electronic books with the launch last November of the Kindle e-book reader.
The device can wirelessly download books, newspapers and other digital content from Amazon. The Kindle has a 6-inch diagonal electronic paper display and uses a high-speed cellular broadband network to let users download content without hooking up to their computers.
Some analysts believe Amazon may soon make a play for the textbook market with its next-generation reader. Sony sells an e-book reader that competes with the Kindle. The Sony device, however, requires a PC for downloading content.
To help understand the social networking and Web 2.0 landscape, InformationWeek has published a paper outlining the market forces at work. Download the report here (registration required).