Twitter is monetized for a month.

Seth Goldstein, CEO and co-founder of socialmedia.com, said that next month Twitter would launch an advertising platform that was described at the IAB Annual Leadership Meeting 2010 as "a new generation of interactive advertising."

According to Anamitra Banerji, a Twitter employee responsible for monetization issues, the platform is currently in the testing phase.
At the conference, he demonstrated a graph showing the number of tweets during the final of the National Football League. One line showed tweets about the game itself, the second about brands and ads shown during the broadcast. The second line is not so far behind the first.
“On Twitter, people constantly discuss brands, share their opinions about them. Why don't the brands themselves enter this conversation? ”

Many twitter messages are accompanied by hash tags. In particular, the advertising display system will be based on them in order to choose what to advertise to this or that user. Hash tags are not the only base on which the system relies. However, other details are not yet known.
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It is noted that the emphasis is placed on a particularly high relevance of the displayed advertising, so that users "will not even think of it as advertising."
More details are expected later.