Last name, First name (username). “Entire tweet content in quotation marks, reproduced exactly as posted.” Date, time. Tweet.
Allen, Joshua (fireland). “Been on hold so long I can’t remember who I called. I have a credit card out and my pants off but that doesn’t really narrow it down much.” 30 June 2009, 1:08 p.m. Tweet.
Note that the date and time should reflect what you see when you view the Tweet on Twitter. (Always go to Twitter to ensure accuracy, rather than using a Tweet that someone else has inserted into an article or website.)