Twitter spam, the under loved and Qwitters
You can't technically be spammed via Twitter in that you need to agree to follow someone to get the junk they may be tweeting.
But that doesn't stop the spammers, the 210,000 underemployed who have self described themselves as social media experts, or the 'under loved' from trying to get you to follow them on twitter.
How do they do it? They look for the Twitter users who have a large number of followers (3,000 or more) and start following them. Doesn't matter that person they seek to follow on Twitter is not sharing things of interest to them or is not a person they want to get to know better.
Why do they start following random people? They're hoping that they get followed in return.
What happens if you don't follow them back? They stop following you. They become Qwitters.
You only get to follow about 2,000 people on Twitter unless you have an equivalent number following you. When you're following for the sake of getting followers you just can't afford to follow people who don't follow back.
For awhile I bought into this game. I followed Guy Kawasaki's advice that if you wanted more people to follow you on Twitter you followed everyone who followed you.
Guy's logic, courtesy of Robert Scoble:
It's courteous to do so and because when you do, some people will respond to you and eveyone who follows them will see this--which is more exposure for you.
In addition, like a lot of other folks, I wanted more followers. It made me feel good, I guess more important, and it certainly made more people follow me. When I broke into the top 1,800 most followed on Twitter I'd have 50 or 60 new people a day following me.
But at the risk of being unloved and looking less important, I made the decision not to follow everyone who follows me. If that causes the social media expert who teaches yoga while on the beach in Malibu to stop following me after I don't follow them back in a couple days, I'll suffer through the pain.
Maybe I'll be viewed as a Twitter snob or God forbid I'll fall off page one of LexTweet, our community of legal professionals using Twitter. I'll suck it up and survive some how.
Going forward, I'll follow those who it makes sense for me to follow. If it looks the person may have an interest in the things I share via Twitter or an interest in getting to me know me, I'll follow them.
I'll follow those who have an interest in the things I have an interest in - the law, the media, blogs, social media, journalism, PR, sports, fellow CEO's & leaders of innovative companies, my competition, family... Heck, like everyone my interests are pretty diverse. And I'll of course I'll follow those people I want to get to know.
You'll need to develop your own style as to who to follow. I'm just not certain you want to encourage all the spammers, so called social media experts, and the under loved to keep stalking people on Twitter by needlessly following them.

I love how you (correctly) point out that spamming isn't technically possible on Twitter, then in the next paragraph talk about spammers.
I thought spammers were people who spam, so if you can't spam, how can someone be a spammer?
People should be free to decide whom they want to follow or not, and it's really nobody else's concern.
As a relative newcomer to Twitter it isn't always that easy to figure out who to follow (and why on earth people are choosing to follow you sometimes!) and it is useful to get some perspective on the subject.
My general policy is to have a look at recent tweets from new followers and decide whether to follow back along the lines of your post. However, volume of tweets is a big one for me - even if someone is interesting it is difficult to bring myself to follow them if they will end up bombarding my timeline.
Kevin -
I think early on it was the norm to follow-back everyone who followed you. But that paradigm has clearly changed.
Are you going follow-back that spam follower? No, so that 1 to 1 relationship is gone.
Do you expect a celebrity to follow me back? Is Lance Armstrong going to follow? No.
So you follow people that interest you and some of them will follow you back because you interest them. Don't take the lack of following or stops following you personally.
If you want to be followed, make sure you put up a picture, tell us about yourself (in less than 140 characters) and don't protect your tweets.
Since first using twitter, I have made the decision to follow people who I think are going to be interesting and who subsequently add to my twitter experience (sharing links to info. I am interested in, being able to learn something new, making me aware of something that was not on my radar, conversation). I look at twitter as a party I have been invited to and it's up to me to have a good time and make friends.
I don't automatically follow everyone who follows me nor do I expect everyone I follow to do the same. I try to be as interactive as I can with those who do @me but I have followers who have NEVER tried to engage me and continue to stay on the sidelines.
I have purposely NOT followed the main twitter 'stars' just because many do and I figure if they say something relevant I will get the info. in a RT and I don't have to bother with all the noise in my tweet stream.
Doug Cornelius summarized my view of this issue, writing:
I don't usually reciprocally follow others just for the sake of doing so. While I might be happy that a person follows my tweets, unless I ask them directly, it can be very hard, and still entirely speculative, to figure out why they find me or my tweets interesting (excluding the spammers, of course).
In my earliest days of tweeting, I tended to follow everyone who followed me. That quickly became overwhelming, so I became more selective.
I don't take it personally when someone I follow does not follow me in return. On the other hand, I'll admit I sometimes feel flattered and honored when I someone I consider a really high-powered person follows me. Even then, I don't let my head get too big because, again, without asking them directly, I can guess why they found my tweets interesting, but I still won't be certain.
Followers come and followers go. I think that's part of the nature of micro-blogging.
If I remember that I'm using Twitter as a networking and communication tool, I'll avoid considering it as some high school-esque popularity contest, and I'll gain more benefits from tweeting.
When someone follows me, my general policy is to check out their bio and recent posts, and if it looks like there's something of interest there then I'll usually follow them back. If I don't follow them back, it's nothing personal. It's just difficult enough as it is to keep up with the people I do follow. I couldn't imagine trying to keep up with thousands.
But if it's obvious they're spamming or just using Twitter to push a product then I block them (fine line there - sometimes I think people can 'spam' unintentionally, when really they just don't get it, and haven't taken the time to figure it out). I've never really understood the value in using Twitter for spam. For the effort the person on the other end has to put into it, I can't imagine there'd be much return. But like email spam there must be enough suckers out there to make it worth their while.
I'd like to think those who do follow me genuinely care about what I might have to say, but my "Twitter rank" doesn't mean a lot to me. For me, Twitter is simply one of several tools I use to enhance my online experience.
Thank you for this. When I started with twitter I did it the wrong way and I guess I was spamming. Now I am trying to figure out how to do it the right way. This does help. Thanks again
I wish you had written this post before I had joined twitter. I put a side 10 mins. of my day just for unfollowing.