I prepared summaries for my upcoming Search & Social Double Whammy seminar on May 2 in Burlington, MA describing the “big three” social networks: Facebook, LinkedIn and Twitter. My goal was to describe in plain English the way these networks provide value to their users and the metaphors they use for interaction. Perhaps you’ll find these basic explanations useful in some context. And if I’ve missed or misstated anything, I’d appreciate your corrections.

Facebook & LinkedIn

The two most popular social networks – Facebook and LinkedIn – use similar tools and metaphors to provide strikingly different utility.

Both are based upon a foundation of personal profiles and “friends,” which LinkedIn calls “connections.” Profiles are online identities that define members’ backgrounds and interests and reflect their activities and contributions to the community. The more active members are in the community, the greater their influence and the richer the interactions with other members.

Friends and connections are persistent relationships between members that require mutual consent to create. Friends can see information about each other that others can’t, and because connections are maintained by the social network rather than by individual members, they outlast job changes, relocations, relationship changes and other disruptions that often cause us to lose contact with others.

The most powerful force created by social networks is the “Power of 130.” The name is derived from the fact that the average Facebook member has 130 Facebook friends. That means that every member’s actions within the community can potentially be communicated to 130 other people through the every-flowing timeline called the “activity stream.”

Marketers can think of these communications as a Web 2.0 version of the classic impression, but social network interactions are potentially much more important because members can comment upon, endorse and share other members’ activities with their own networks. This means that a compelling message can be spread far and wide by the members themselves without investment or active involvement by the person or organization creating the message. Good content sells itself.

Facebook is the overwhelming favorite of business-to-consumer companies because the action is free-wheeling and fun. Good Facebook marketers provide a constant stream of information that provokes conversation and interaction among members. Contests, polls and games work particularly well there.

LinkedIn is a favorite of B2B marketers because its members go there mainly to discuss professional interests. LinkedIn’s roots are in networking for job-seekers, but the service’s active professional discussion groups and useful Answers section have become favorite places for people to gather and share information about their work. LinkedIn also enables members to identify shared connections and to form relationships with others through friends-in-common. This makes LinkedIn a compelling new tool for professional networking and lead generation.

Both Facebook and LinkedIn permit brands to create their own pages to communicate with advocates, build awareness and create persistent relationships. Facebook fan pages focus on conversation with followers while LinkedIn stresses information about the companies. Both services provide great value for brands in very different ways

Understanding Twitter

Twitter is still a mystery to many people. How can rich conversations form when people can only speak 140 characters at a time? It turns out you can say more in 140 characters than you may think, and Twitter’s forced brevity actually encourages people to share information they wouldn’t communicate through long-form media like blogs or even e-mail.

The core feature of Twitter is the activity stream. It’s an endless flow of news, recommendations and observations that create endless opportunities for connection. You can find and engage with people on Twitter whom you could never reach by any other means, and it is arguably the world’s best source of breaking news. It is also a valuable extension of any company’s online presence.

Twitter is a loose-knit social network in which members subscribe to each other’s activity streams in a relationship known as “following.” Unlike Facebook’s friends or LinkedIn’s connections, following does not require the consent of both parties. Anyone can follow anyone else unless explicitly blocked by the person being followed (a rare occurrence).

As members amass more followers, the value they give and take from the network increases. People or brands with large followings can reach a large number of people directly because their messages appear in followers’ activity streams. While the percentage of people who see any individual message may be small, the ease with which messages can be forwarded – or “retweeted” – to others provides ample opportunity for amplification. In fact, a study by ShareThis found that the average retweeted message is shared 18 times.

While the volume of messages on Twitter may seem overwhelming and unmanageable, there are a variety of useful ways for people to organize and discover interesting topics. Members can filter the entire Twitter stream by keywords or “hash tags,” which are category labels members attach to their tweets to associate them with popular topics. Twitter also notifies members by default when their username has been mentioned by another member in a tweet. This notification feature makes Twitter an extraordinarily useful way to find people who may be difficult to reach by e-mail, phone or other media.

Twitter is proving to be particularly valuable for organizing and promoting online and real-world events. Hundreds of virtual chats take place each week around Twitter hash tags in fields ranging from medicine to marketing to aviation. Organizers of physical events frequently ask attendees to use specific hash tags when sharing information about the conference, giving the rest of the world a glimpse into the conversations going on at the live event and promoting it to future attendees. “Tweetups” are physical meetings organized on Twitter using hash tags, and anyone is invited to come. Tweetups can be used for anything from attracting fans to a concert to promoting a book-signing or store opening.

Twitter is evidence of the power of simplicity. Users have adapted and modified this relatively simple publish-and-subscribe service in thousands of creative ways, making Twitter one of the best tools for finding out what’s going on now in a wide range of professional activities and leisure interests.