Twitter is going to target advertising at its users based on what they type--in addition to its existing keyword targeting that delivers ads to users based on what content they consume through the social network.
AllThingsD notes that the service is called "keyword targeting in timelines." The idea is pretty straightforward: If you write a tweet about something, then shortly afterwards you may see an ad that pertains to what you've just shared over Twitter's network. This targeting also includes things you reply to, retweet, or flag as a favorite because these can be considered signals of intent that Twitter can sell as ad slots to its advertising partners.
More interesting is that the service includes "negative sentiment filtering" so if you tweet that you hate something then you won't see a tweet relating to that. This is wonderfully common sense-based, because one can easily imagine a situation where a disgruntled twitterer shares a message about a brand, then was subjected to an ad from that brand, which prompted an even more negative tweet.
Rumors recently suggested that Twitter was attempting to expand its social network by incorporating music discovery services--which is likely to be another future revenue-generating push.
[Image: By Flickr user shirokazan]