What is Yammer? A social media field guide

Foursquare: The one for contant geographic updates

What is it? Foursquare is a smartphone app that lets your friends know where you are, what you're doing, eating, and buying--and lets them see the same of you. The idea is for everyone to mark his territory. Sign in to Foursquare each time you visit Joe's Diner, say, and in time you could be recognized as Joe's most frequent patron. (Congratulations, you're the Mayor of Joe's.) Foursquare allows businesses to send you promotional solicitations that scale in generosity with your rank.

Who uses it? Tech-forward early adopters on iPhones and Androids. A committed Foursquare user likely works in digital media, blog, enjoys prominence on Twitter, usually wears t-shirts, and embraces enthusiastically the inexorable progress of technology.

Do I need it? Absolutely not. Foursquare has never been able to say Why It's Needed. The reason may not exist. Most people don't care enough the occasional free Earls appetizer or manicure discount to continuously broadcast their physical location to their network. Foursquare's primary appeal is to techie people who want to demonstrate techiness to other techie people.

Google+: The one that's trying/failing to improve on Facebook

What is it? Google+ is is Google's answer to Facebook--a more sophisticated social network in which you articulate members of various "circles"--friends, family, workmates, etc.--and use that infrastructure to broadcast updates, share photos, and interact digitally. G+ quickly amassed more than 40 million users, but the figure may be deceiving. Many people either signed up for Google+ accidentally (believing it was a feature of Google's popular Gmail service), or just to check it out. (Your faithful correspondent signed up for his G+ account only to reserve his name, should he want it later. He's never used it.)

Who uses it? Mashable.com, the arbiter of all things social media--says G+ is a hotbed of early adopters, tech luminaries, marketers and businesses. But that's hard to confirm. You can't see if anyone's on G+ unless you use G+, and that first signin chances a chain reaction many people want to avoid.

Do I need it? Wait and see is the prevailing approach to Google+. Few people want to be the first of their cohort on the dance floor.

Pinterest: The one for crafty scrapbookers

What is it? Pinterest calls itself is a "social bookmarking site." The site, launched in March 2010, is a place where users collect and share photos of their favorite events, interests and hobbies. With 104 million usuers, Pinterest is the third-largest social network, behind only Facebook and Twitter.

Who uses it? Women, in the main. Like the Etsy.com, the site appeals to scrapbookers and browsers, those interested in craft, design, and homemade items. The site is soon expanding overseas markets such as France, Germany, and Japan.

Tumblr: The one for bloggers who are over post-postmodernism

What is it? Tumblr, opines Chris Dannen in Fast Company, is blogging, the way blogging was meant to be. He goes on: "Tumblr is a little bit the long lost cousin of its other buzzword counterparts. Like Twitter, it takes the now-antiquated medium of the blog and refreshes it with a new format, a centralized platform, and more interactivity. Like Facebook, it's yet another face on your online personality, a scrap book of the Web-you. In one sentence, Tumblr is a blogging platform that makes it easier to post video, audio, words, social bookmarks, photos, and even other people's blog posts into your blog, and share it with other people. Instead of having to upload things to YouTube, Delicious or Flickr, or create your own WordPress database before posting things, you can put your media directly into Tumblr from your computer or mobile phone."

Who uses it? Hipsters and creative types. Those raised on the Internet, for whom simple blogging or tweeting is frustratingly straightforward.

Twitter: The one for short updates to and from the hive mind

What is it? Twitter is a microblogging service that enables users to send and read text-based posts, or "tweets," of up to 140 characters. If that's less than clear, think about it like this: It's millions of people, all over the world, on their computers and smartphones, cracking short and wise about updates about what they're doing, thinking, reading, and watching.

Who uses it? Celebrities, athletes, marketers, journalists--anyone with a need or desire to feel in touch and up to date, every moment of the day.

Do I need it? Twitter has enabled, more than any technology that preceded it, your entry into the chattering collective mind of humanity. There are a bunch of good business reasons for using the service, but they're too tedious to talk about. Need it? No. But Twitter offers something interesting, and new. In simultaneously talking to (and eavesdropping on) everyone and no one, you feel an all-at-once surge of the connection and loneliness of modern life.