Whereas with email the SPAMMER needs to specifically target you and therefore the SPAM is clearly intentional, with Twitter it just gets broadcasted into the Twitter ether. If you happen to be following the SPAMMER, you see the tweet.
Someone you follow might be putting out some really good posts and some really SPAM-like posts. Since, in Twitter, you decide to follow the person and then get everything they post; some tweets will have higher value to you than others. Some will skirt the line of SPAM and others will go over it (in your eyes). This is not absolute as other followers likely draw a completely different line. Remember, there are plenty of people out there who enjoy watching QVC. If the noise/SPAM to value ratio goes up too high for you, you may eventually unfollow. However, since that takes a purposeful act, many may suffer the noise until it becomes unbearable.
There are some new Twitter apps that are negatively impacting the noise ratio.
- Chat clients where chatters are filtering Twitter messages for a certain hashtag. Their chat messages auto-include that hashtag. eg. TweetChat.com While the chat makes total sense to the participants. Other followers are getting bombarded by frequent often non-sensical messages.
- Games where the game is actually played with Twitter messages. eg. playspymaster.com
I am far from calling out that the sky is falling. There are also plenty of applications positively impacting the noise ratio.
- Tweetdeck is probably the most popular
- Seesmic Desktop is catching up quickly
- It looks there is a new entrant Mixero with similar features and increased usability
I expect the Twitter stream to get more and more noisy. Likely both types of apps will grow significantly.