...and on the content your are writing, and the sort of influence you want to have. I know of several Tamil writers online who have dedicated following. If you want to influence online Tamil community, then I don't see the point of writing in English or Spanish.
There is probably some purpose for this study, but it is definitely lost in the blurb written above. While it is obvious that most of the internet talks in English, the ability to weild influence online is not just dependent on the language you are writing in. With instant page translations, good amount of the written content is accessible to larger section of audience, regardless of the language it is written in.