He is right . the main font is "Lucida Grande".
Check this is the universal link for CSS styles appled for Twitter.
I'm not sure if this is the exact font they used, but it looks to me like the "Chickens" font.
Good luck :)
-Whitney Sasser (:
It's actually a combination of arista and arista light.
there are two fonts that look very close. pico black and arista. if you want to view test and download all the fonts that look close to the twitter font check out this site.
Font "Pico alphabet black" + small grafical changes at "e" and "t".
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them