All of these different social networks that you can join - Twitter, Facebook, MySpace, or Last.FM, for example - could be thought of as neighborhood pubs. You could decide to go barhopping every time you go out, you could decide that you only like one of them and never visit any others, or you could frequent a few different places fairly regularly. None of these choices is a 'wrong' choice, and isn't even necessarily decided on by the quality of the location itself. The pubs you go to or the social networking sites that you join & frequent are actually more likely to be decided based on where your friends are.
Maybe MySpace is that seedy dive bar of the internet you keep getting dragged to, wondering what others see in the place. You might continue to go back every once in a while to catch up with any friends that are still kicking around, but your visits are less and less frequent until/unless they add or update features (keeping with the pub analogy; add a drink special, or start getting decent bands). Maybe, for your group of friends, Facebook is the new popular bar that everyone is at all of the time now... Twitter might still be considered the trendy new club that your friends are hesitant to go to because they only serve beer (in 250ml glasses?)...
Also, don't forget that email and IM conversations fall into this categorization as well - you might decide to stay in or go to a friends house to have some drinks for the evening. This is compareable to an IM chat or an email conversation; closed to the public, and likely only involving a few people at a time. Just as I have friends who won't stay in & like to go to a new place each time they go out, I have other friends who always stay in & are rarely found waiting for a drink at the bar.
So, deciding which social networks to join should be less a question of whether or not you should be on the next latest & greatest social networking site - it should be more a question of where your friends are, and what type of environment you feel like participating in. If all of your friends got up in the middle of a house party and decided to head down to one of the local pubs, would you hang back at the house alone ... would you go with them ... or would you even go out on your own to a totally different spot and see who you met there? There isn't a wrong answer as to which social networking site you're using, just as there isn't a wrong answer as to what you'd like to do tonight. The answer will revolve around where your friends are, and if you happen to want to be there as well or if you want to branch out and see what else is going on.
...and if you're a business owner, you can go ahead and re-read this post substituting 'customer' each time you come across the word 'friend'. I suppose the pub analogy only works if you do business like you're a character in Mad Men, but I think that the "be where your customers are" idea still fits just fine.
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