SFGate.com, the online version of the San Francisco Chronicle and the leading news and information Web site for the San Francisco Bay Area, has made good use of social media tools to create a large and vibrant community around its news and opinion content. Over the last few months the Latin music site Batanga.com relaunched its site with an ambitious new social experience, and the football portal NFL.com provided an extremely popular draft "War Room" forum for its fans. These are very successful, highly energetic, audience-driven communities that are delivering real value to the organizations that sponsor them by capturing and channeling the potentially lucrative passions of their core audiences.

Despite their successes, these pioneering organizations understand that their social sites are still islands in a large ocean. As with any island, the passion and energy these organizations have worked hard to generate and sustain generally ends at the shoreline. And like even the most beautiful islands in a big ocean, successful social sites can sometimes remain a well-kept secret from the larger Web. As a result, attracting new site visitors, each of whom could bring additional passion and energy, is often an expensive and incremental process.

Social sites dedicated to publishing, shopping, entertainment, recreation, health and fitness, and sports all let like-minded people collect around a brand, a topic, or a lifestyle. There, they share ideas, views, and experiences. And yet, as appealing as these sites are -- and based on their growth rates, they clearly appeal to great crowds of people -- they nonetheless frustrate one very basic human drive. Because while it's possible to make friends at a social site, we usually find that in the online world as in the real world, most of our real friends are somewhere else.

Ironically, just over the horizon from these vibrant island communities lie popular social destinations like Facebook, LinkedIn and Twitter. This is where Web users already hang out with their friends online. And even more ironically, while people go to social sites that address their particular passions for music or sports, you can be sure that they also talk about music and sports with their friends at cocktail parties and on Facebook and Twitter.

The challenge for owners of social sites is obvious. It's expensive to get word of good social site experiences "off the island" to attract new participants. Often each participant in a social site has a core group of friends that has yet to make it to there, where their friend is so engaged and so pleased with the experience. And while it may be tempting to move the social site to Facebook, where all those friends are, the fact is that social sites are so successful because they surround compelling content with community. Social destinations are designed to host broad friend-focused activities. They have generally failed as places to put in-depth content.

Each of these two forms of online social experience is important. Each is growing in popularity, each serves important goals, and each will endure. Social destinations create and serve a need to be in contact with friends online. For social sites, the value to their owners is clear: more traffic, more engagement, more conversions, and more revenue.

I believe the answer to this challenge lies in what we call social bridging. Social bridging lets owners of social sites connect with the Web's popular social destinations. It makes it easy for enthusiastic members of social sites to communicate their enthusiasm to their friends on Facebook, Twitter and similar destinations. For new visitors, it simplifies the entire process of joining and getting established at social sites.

SFGate.com is doing some very exciting things with social bridging. This publisher is already connecting its social site to popular social destinations like Facebook and Twitter. It works like this: After registering free of charge and creating a profile on SFGate.com, readers can choose to extend a bridge to their friends on Facebook or Twitter. On SFGate.com, when these people read news that interests them and submit article comments, these comments can be automatically uploaded to the reader's chosen social destinations, broadcasting their opinion to those places as well. All of the reader's followers are made aware of the engaging comment the reader has found at SFGate.com. And, serving as a two-way passage, this same bridge lets readers invite their Facebook friends to come to SFGate.com.

The benefit to SFGate.com's readers is that they can share their experience at the website with friends who are not on the island, and they can even encourage their friends to join them on the island itself, bringing with them their identities and, eventually, their friends. The benefit to the publication is that its readers are promoting the site to corners of the social Web where the website doesn't reach, and even attracting new readers at practically no cost to the publication. And having been invited by trusted friends, these readers are usually highly engaged once they arrive.

Since connecting with Facebook in December 2008, SFGate's referrals have grown exponentially; upwards of more than 400 percent. These results demonstrate that leveraging social bridging to support two-way traffic and unify the social web really is taking off, and the consumer response is unprecedented.

Social bridging is real. It relies on a mix of technology and online community management best practices. The technologies are, and will continue to be, based on a mix of software industry standards and social media software vendor offerings. And the best practices help social site owners to create online social structures and relationships that more closely mirror our offline lives. And while social bridging is still in its infancy, it suggests a future where our online identities, relationships and conversations will cross the Web effortlessly.