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Twitter is a popularity contest, but it doesn't really matter if you're popular -- you can still go to the party. And partying there is the brand of twitters I like to call Twitter Whores.
Some of them really are whores -- porn sites trying to get you to dig into your trousers and open your wallet to see a little boobage (and more, obviously). Some are just people who want lots of followers to try to sell those followers other products and services. And while you would never talk to these people if they tried to cold call you, they have been successful getting followers simply by following, because everyone wants to be more popular on Twitter.
These aren't your friends or your colleagues, and as we get better filters, they aren't being as successful in getting followers. I'll let them follow me (unless I really hate their avatar image) but I'm not going to follow them back.
But they are finally figuring out that they don't need you to follow them to reach you on Twitter -- instead, they just randomly mention you. I'll always look for mentions of my name, so I'll see their posting. And, they can be vague enough to get you to click through to their websites with a "@bissell: did you see this? http://somelink/xyz"
It's completely automated (just scan for Twitter handles and post your tweet), easy to scale (you can set up new accounts faster than Twitter can shut you down) and you don't need anyone to follow you. And with shortened URLs, it's hard to know where you're going until you land -- nearly guaranteed traffic generation.
Regardless of whether anyone buys the Viagra or porn membership on the other end of these links isn't really even the point. It's the theme I keep getting back to -- as our social lives change and use more tools to broaden who we consider friends, our filters don't keep up, and there will always be clever people out there figuring out ways to get past your defenses and into your pants.