What language is your internet? Part II

Language choice online seems to depend on a number of factors. Our audience is certainly one of these. But what do you do if you and your audience share several languages? Does the medium influence which language you pick?
Sarah Pasfield-Neofitou reports an interesting case: An Australian (native speaker of English) and a Japanese (native speaker of Japanese) who kept contact through two different social networks - and used (mostly) English on one of those (Facebook), and (mostly) Japanese on the other (Mixi), even though they discussed the same kind of content in both contexts!
On Twitter, I use mostly English, even with my German speaking colleagues (thanks for indulging me, folks!). Now, do I do this because I perceive Twitter to be an English domain, as Pasfield-Neofitou suggests, that is, because I strongly associate use of Twitter with English, or do I do so because the messages I exchange with colleagues are also intended for a secondary audience of "Twitter bystanders" who may not speak German (overall, most of my Twitter followers tweet in English)? Fascinating question :-)