Is anyone in your (extended) family needing a primer on (Web- and phone-based) social networking, maybe to understand what his great-granddaughter finds so compelling about "this Facebook thing?" If so, David Pogue at the New York Times gets really, really basic – in both a cute little video and brief written explanations on this page – of Facebook, LinkedIn, Twitter, FourSquare, and Yelp. The five services each represent basic categories of social networking (though it's important to note they're not all popular with teens; e.g., Pew/Internet reports that only 8% of teen Net users use Twitter):

* Facebook as the big general social site, or social utility (with a growing proportion of these services' users accessing them via mobile phone); of course, FB's by far the biggest, with nearly 500 million users worldwide and now getting more Web traffic even than Google.
* LinkedIn as the biggest and probably oldest professional social network site, where people's social networks can provide leads on jobs or potential employees or serve as professional learning networks (PLNs)
* Twitter as the most popular micro social network service (almost a hybrid of Web and phone texting, though Twitter has also been described as "micro-blogging"). Both Twitter and SMS texting restrict message lengths ("tweets" are limited to 140 characters and text messages to 160); the difference is, texts are usually little links in a chain of whole (text) conversations, while tweets are more often mini status updates (but they can be parts of conversations too). [See this Los Angeles Times blog post for how the mobile carriers arrived at that character limit.]
* Foursquare as the most well-established location-specific (or GPS-enabled) social networking on cellphones, whereby people send shout-outs about their whereabouts, such as a favorite Starbucks or sushi bar to their social networks. Other kinds track your whereabouts in realtime for specified periods of time (like glympse, e.g., for parents when they want to be sure young drivers get somewhere without detours) or indefinitely (as with loopt for friends and family to find out who's nearby and wants to get together.
* Yelp as socially enabled Web- and phone-based reviews of restaurants, hotels, museums, and other places in your own location or cities you're traveling to when you need them – think of restaurant reviews "of the people, for the people" – like Amazon's book reviews, the most anonymous of all these categories.

But the number of social-networking categories is growing. There's "social networking" or really just socializing and communicating in virtual worlds, multiplayer online games, social gaming services, and videogame worlds like Xbox Live. I think a lot of people who grew up in the mass-media era think that at some point people will move on from services like MySpace, Twitter, or Facebook en masse, or flock from Facebook to Twitter. That's just mass-media think. But what's happening is, the kinds or ways of digital socializing are multiplying, people of all ages are adding new tools as they find them useful, and how they find them useful is very individual. For example, the research shows that, though texting by teens is growing exponentially, it's not replacing online social networking (see this from Pew Internet on the various social tools teens use)