Plaxo, which makes software for PC users to keep address books up-to-date, said on Sunday it is helping millions of members open up their online datebooks to build social networks like MySpace or Facebook.

In a major comeback push by the 6-year-old company, which Silicon Valley insiders see as a forerunner of the social-network craze, Plaxo has created the first Web service to share data between major address and calendar programs.

With Plaxo 3.0, as the new service is known, consumers can synchronize address books and calendar data locked up inside Microsoft Outlook, Googleand Yahoo services, Apple Macintosh computers, Mozilla Thunderbird e-mail and many mobile phones.

Now Plaxo is going head to head with social networks, which rather than forming links between computer address books link people online through their shared media interests -- based on what they write or the photos and video they choose to share.

Plaxo plans to let friends keep track of their contacts' Web surfing habits, paralleling a feature fuelling the rapid rise of Facebook, which allows users of all ages to create private networks to connect to their immediate friends. Watching and learning what your friends are doing on the Web -- and being watched yourself -- is today's hottest online trend.

The most important things happening right now on the Web are about people's relationships with other people, Chief Executive Ben Golub said in an interview. There is no reason to segment your relationships with others based on which applications you use.

Plaxo previously enabled individual users to stay in touch by automatically sharing data between contacts' address books. It has signed up more than 15 million registered users to date, although the number of active users is unknown.

Plaxo functions like a network of address books. When a phone number of someone's contact changes it automatically updates in contact address books, assuming the contact gives permission via e-mail.

We tackled the hard problem of making synchronization that just works, Plaxo Chief Executive Ben Golub said in an interview. Our role in this is to be the Switzerland of personal information.

KNOWING MORE ABOUT WHO YOU KNOW

The move involves synchronizing all the scheduling and address information an individual has scattered across computers, mobile phones and online accounts and linking up this data to personalized Web pages so that friends can stay abreast of Web activities the user lets them know about.

He who syncs best, wins, Jupiter Research analyst David Card said. It's not something you can get from Google yet. Plaxo's approach has got good potential.

Plaxo Pulse, as the social network profile site is called, uses Really Simple Syndication (RSS) feeds to pull in Flickr photos, blog posts, Amazon.com Wish Lists, and Plaxo contact changes from friends who permit sharing personal information.

Plaxo 3.0, which is the result of a quiet year-long reworking of Plaxo software, is available in seven languages -- English and, for the first time, versions in French, German, Japanese, Portuguese, Simplified Chinese and Spanish.

It is in public test mode and can be found at http://www.plaxo.com.

Instead of needing to download Plaxo software to one's computer or phone, Plaxo 3.0 is delivered as a service over the Web.

In a twist on the notion that all humanity is connected by only six degrees of social separation, Plaxo has aimed to organize users' ties to people in real life -- those in one's address book -- while social networks connect their users to second and third-level connections -- friends of friends.

Plaxo 3.0 can connect Google Calendar (and shortly Gmail's address book) with Microsoft Outlook, Outlook Express, Vista Mail or Hotmail, Yahoo Mail and Calendar, Apple services like iCal and Address Book, AOL AIM and LinkedIn Premium version.

For mobile phones, a WAP 2.0 version for phone browsers is available at http://m.plaxo.com.

U.S. CDMA mobile phones -- Verizon and Sprint users -- can download Plaxo. over the next several months modern Java-based handsets from companies like AT&T and T-Mobile will be ready to run Plaxo. Mobile phones from international operators will also follow, officials said.

The Plaxo service is free. A premium version that costs $49.95 per year offers features like automatic data recovery and eliminating duplicate contacts, company executives said.

Plaxo has taken $28 million in funding to date from backers including Sequoia Capital and DAG Ventures.

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