In fact, the 7th edition (2011) already has a slightly dusty feel to it, with its coverage of things like CD-ROMs. Should you ever need to cite a tweet, the Modern Language Association of America (a major group of academics in the humanities) has published guidance in its style handbook: ‘Last name, first name (user name). “The tweet in its entirety.” Date, Time. Tweet.’ To use one of our heaviest users of Twitter as an example, the citation would be (use your own date format, if you want):

Smith, Michael (MichaelSmithYYZ). “Important case from ONCA – joint and several #liability for #negligence & offers to settle in multiple defendant cases http: //www.ontariocourts.on. ca/decisions/2012/2012ONCA0025.htm”. 26 January 2012, 9.58 am. Tweet.  

[Link available here].