A lot of Twitter users are unaware of this but Twitter did run Adsense ads on their webpages. (see an old screenshot of this page from Google Cache above).
The Adsense Ads were shown only on the individual tweets and not on public timelines or profile pages.
Now Twitter has removed these Adsense ads - may they were not performing as well as Ev and others at Obvious would have expected them to.
1. Most tweets are short sentences and therefore contextual advertising will not work in absence of text.
2. 234x60 is not the best performing Adsense format. Also, the Google Ads background were not blending with the page background color.
Or maybe, Twitter founders are exploring other options to monetize twitter. Steve is speculating that Twitter may be on the block soon.
Also read the Twitter Joke.