Different social-networking sites promote different types of interactions and activities. To understand why people use social-networking sites, it's helpful to first break down these sites into some general categories.
Community Social-networking Sites
This is a broad term for any social-networking site whose purpose is to help users identify and enlarge their social networks by searching for existing contacts and finding new ones.
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Some of these community sites aim for a particular demographic: LinkedIn attracts working professionals; the original Facebook stuck to college kids; Black Planet, Asian Avenue and Mi Gente target racial and ethnic affiliations. But other sites, like MySpace, Yahoo 360 and Hi5 are general community sites in which users can form smaller groups based on interests or demographics.
Media Sharing Sites
These are sites where the main activities are uploading photos and videos and viewing other user's photos and videos. You don't have to be a member of the site to look at other people's pictures and movies, but you have to join to upload your own.
Once you've joined, you can invite other users to be your friend, at which point they'll be added to your page as links with profile photos. YouTube is the most popular video-sharing site and Flickr is one of the best-known photo-sharing sites. We should mention that many of the general community sites include media-sharing elements, like the ability to post pictures, videos and music on personal profiles.
Social Bookmarking Sites
Bookmarking is the practice of saving a link to a Web site in your Web browser. Social bookmarking, on the other hand, is saving that same link to a special social bookmarking Web site where other people can see what you're bookmarking.
When you save a link to a social bookmarking Web site, you tag it with as many keywords as you like. Other people can look up bookmarks by tags, or they can browse by the most popular or most recent bookmarks.
If you like the kind of links someone else is collecting, you can add that person to your network and receive notifications when they add a new bookmark. Examples of popular social bookmarking sites are del.icio.us, Digg and Furl.
Blogging Social Networks
These are social-networking sites, which bring together individuals and their blogs. Instead of a simple profile, each user keeps a running Weblog on topics of their choice. Sites like Livejournal, Blogger and Xanga allow users to search and browse for interesting blogs. When you find a kindred spirit, add his blog to your list of friends. You can form communities with other bloggers.
Music Social Networks
Sites like Pandora, Last.Fm and iLike are a new breed of social-networking sites based on sharing music with friends. Pandora and Last.Fm work like streaming radio stations. You enter an artist name and the Web site creates a play list of songs that it thinks you'll like. You can play the full list in your browser and even rate the songs so that the Web site gets to know your tastes better. You can share your play lists and favorite songs with "friends" who are part of your network.
Many of the more than 300 social-networking sites fall outside these five categories. But no matter the specific audience or goal of a social-networking site, they can all be powerful social-networking tools by strengthening our connections.
Find out more about how social-networking sites are bridges to friendship.