To help add a social networking feel for Microsoft Outlook e-mail users, San Francisco startup Xobni has launched a beta service aimed at helping organize their e-mail inboxes by business and personal relationships.

As Outlook remains the most common program used for e-mail in corporate America, Xobni (available for download here) aims to make it easier for users to find e-mail addresses and phone numbers faster than simply relying on Outlook alone.

"Today is an exciting day for everyone at Xobni," the official Xobni blog reports. "After seven months of invite-only beta, and over two years of product development, we're proud to announce that Xobni is now publicly available for anyone to download."

Operating as a sidebar, Xobni offers fast e-mail searching, threaded conversations, while showing users relationships, communication habits and context around select e-mail messages.

Using inbox data mining methods, Xobni collects names, phone numbers and mailing addresses, then adds them to users' sidebar. For example, clicking on an e-mail from a co-worker will provide the user with that co-worker's personal data, conversation statistics, other conversations, and transferred attachments such as photos and PDFs.

Ideally, users will be able to find archived e-mails and attachments without having to use Outlook's search function, which can become bogged down over time.

Xobni originally launched during the TechCrunch40 Conference last September, where thousands of users downloaded the software before company organizers removed the download. The closed beta had 50,000 initial users, although more than 140,000 people signed up for the service.

The three co-founders, Jeff Bonforte, Matt Brezina, and Adam Smith, collected more than $4 million in funds, with money coming from Vinod Khosla, one of the co-founders of Sun Microsystems, and Niklas Zennstrom, one of the people responsible for Skype.

Microsoft showed interest in buying Xobni, with Bill Gates claiming the service is "the next generation of social networking," but the two parties were unable to work out a deal.

It's possible the service will be expanded outside of Outlook, including to Web-based Internet services Gmail and Yahoo Mail, along with social networking sites Facebook and LinkedIn. But Xobni will face stiff competition in the e-mail field, especially from Yahoo and Google, with both companies developing what they hope will be the next-generation of e-mail.