Twitter is proving to be a fantastic resource for linguists and you can see several examples of what's been done with it in these previous blog posts.
More recently, researchers at UCL have been looking at what tweets in London reveal about patterns of language use. The people looking at it, Ed Manley and James Cheshire, aren't linguists but an engineer and spatial analysis lecturer (eh?) so they're more interested in where things happen rather than necessarily exploring why - which might be more interesting for English Language students - but it raises good questions about why certain languages don't appear as often as might be expected- Bengali and Somali, for example - while others do make an appearance - Haitian Creole, Basque and Swahili -among them.
But what might be of even greater interest to A level English Language students is how Twitter is proving to be a source of fantastic data on gender and language, and what we might term communities of practice: groups of people that you "do" language with in your various day to day activities and whose language styles influence your own.
This excellent article by Ben Zimmer in the Boston Globe gives you a clear introduction to what Twitter is offering linguists and you can see more of the work of Tyler Schnoebelen and his colleagues in this powerpoint of their presentation to #nwav41 (Warning! Contains advanced statistics to boggle the mind).
In other Twitter-related research, this link to a paper by Rebecca Maybaum gives you a glimpse of how Twitter can be used to track evolving slang: in this case, the words used to describe people on Twitter. Tweeps? Twiends? Tweethearts?