Facebook is the most popular, but like all fads, I think that the popularity will slowly fade. People that used to use it daily, now rarely login at all anymore. Same thing with MySpace.
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I am not that social guy but I try be on FaceBook whenever I can. (Few hours a week)
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Facebook’s website design is straightforward; it has few graphics or decoration, but they managed to create a uniform utilitarian interface that most users adore. Although you can’t customize skins or change your color scheme, you can rearrange the position of some of your information boxes on your profile by simply dragging and dropping them to other spots on the screen.
MySpace used to be my favorite, but Facebook seems to have gone to the top. A lot of people I know or have known seem to pop up on Facebook. It's good way to promote your website as well.
Twitter, but I don't consider it as social networking, but as a multi-source news-line... at least I connect only to profiles that have something meaningful to say.
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