Twitter reminds me of a RSS feed reader. If you look the the stream of tweets in the same way you look at a RSS feed some things start to make sense to me.
Twitter is like a personal RSS feed. I currently subscribe to over 200 RSS feeds in my feed reader, it is impossible to work out what's important when you have well over 1000 items a day appear in your inbox. Twitter is the same, the more people you follow, the more noise there is in your feed, the more noise, the harder it is to work out what is important.
So to manage the RSS feeds you use a feedreader, automatically filter feeds, tag important ones, save items to read later, etc. Twitter needs to be managed the same way, hence the secondary market.
Applications such as tweet deck and web services such as peoplebrowsr.com take the twitter feed and let you manage it, these "secondary market" players have a fantastic opportunity, they make sense of twitter!
So what's the opportunity in the secondary market? Fundamentally it looks like the opportunity is in removing the noise from twitter, so you can receive the signal. What you do with the signal is then up to you. There are any number of articles that discuss the social networking, customer service, citizen news, marketing, presence indicator, take your pick.
How will secondary market players make money? Take the feed and filter it, the more intuitive the filter is, the more value it can provide, and hence, in the future can be charged for (primarily in the corporate space for marketing and customer service).
Have fun twittering, and good luck with the signal to noise ratio!
