There is now an official Modern Language Association standard for referencing tweets: “How do I cite a tweet?“:
Begin the entry in the works-cited list with the author’s real name and, in parentheses, user name, if both are known and they differ. If only the user name is known, give it alone.
Next provide the entire text of the tweet in quotation marks, without changing the capitalization. Conclude the entry with the date and time of the message and the medium of publication (Tweet). For example:
Athar, Sohaib (ReallyVirtual). “Helicopter hovering above Abbottabad at 1AM (is a rare event).” 1 May 2011, 3:58 p.m. Tweet.
What strikes me as absolutely odd is that the standard does not require a link to the tweet. While this is completely in line with their other standards, as citing blogs and websites also do not require a URL, both tweets and blogs, and most websites due to the increasing use of CMS-systems, use permalinks which makes them absolutely perfect for referencing. With born-digital material increasingly becoming citable material I hope the MLA is at least discussing the option of including the source of this born-digital material.
And if we’re starting to consider to cite natively digital material according to their own medium-specific features instead of trying to translate them to print features, I would also nominate to include the @-symbol with the username.