Before I answer the question, I need to tell you a little bit about Twitter. Twitter is a mass communications tool that's not used exclusively for answering the question of "What are you doing?". There are people on Twitter having long conversations, with most of it being public.
People send and receive "tweets" with 3 different instant messengers (GTalk, LiveJournal and Jabber) and SMS text messaging. People who have mobile phone services including at least one of these instant messengers have the best of both worlds on their mobile phones (and probably save a lot on SMS charges). Some people send and receive the tweets using the web interface, but tend to move away from it when they discover other options.
The most difficult thing, that I've seen, is keeping up with specific topics when you're following a lot of people on Twitter. The web interface, as well as specific Twitter desktop clients such as twhirl, allow you to go through the archives for yourself and your friends, or just about anyone you have the user name for. There are other options for getting the messages, even when you aren't actually using Twitter. You can check your FriendFeed (if you use it) or subscribe to various RSS feeds from the web interface. There are probably more ways that I haven't even discovered yet.
What kind of marketing am I talking about?
More than anything, you're marketing yourself. You're promoting your thoughts and ideas and possibly even your brand. The people following you are expecting it, in one form or another, or they wouldn't be following you in the first place.
One of the first things I noticed when I started following people on Twitter were the people mentioning websites and blog articles in their messages and including short URLs from services like TinyURL and Tweetburner (the short URL of http://twurl.nl, integrated with Twhirl, redirects to Tweetburner on the web). I recognized it immediately as a form of marketing.
Self-Promotion
If the tactics of prominent bloggers like Darren Rowse (ProBlogger) and Guy Kawasaki (Guy Kawasaki) are examined, you'll see that they promote themselves, their blogs and their services every time they're using Twitter. This isn't a bad thing because they promote other blogs and other people's services just as often. This is how you self-promote without irritating your followers.
Bloggers like Maki (DoshDosh) are testing it with social websites like Digg and bloggers like Globetrotteri (My Several Worlds) are testing it as a blog posting tool. I'm sure there are other bloggers testing it for other purposes. In most cases, they need help from their followers to ensure their testing sessions aren't really just exercises in futility.
Short URL click-tracking
I mentioned Tweetburner for a very specific reason. Tweetburner tracks and displays all Twitter messages from the public timeline that use their short URL (again it's http://twurl.nl from Twitter or Twhirl), not just who you follow or who follows you. They display the "Tweets with most popular Twurls in the last hour" and the "Top 10 URLs with most clicks in the last hour". You can click the short URL through Twitter or at the Tweetburner website. Either way, the clicks are counted. In my opinion, it's a valuable service that could drive a lot of traffic if it's not abused.
Can Twitter be used for marketing?
Finally answering the question, yes, it can. In fact, it's already being used for that purpose and I've only told you how I've seen it used. I'm sure it could be used for much more.
Can you think of any other unobtrusive marketing tactics that could be used with Twitter?
