According to Nicholas Ostler in The Last Lingua Franca: English Until the Return of Babel  (2010):

The online communities that use languages other than English have grown meteorically in the first decade of the twenty-first century. From 2000 to 2009, the fastest growing languages on the Net (in numbers of users) were Arabic (twentyfold), Chinese (twelvefold), Portuguese (ninefold), Spanish (sevenfold), and French (sixfold). (pp. 263-264)

And according to the infographic below, Chinese will become the dominant internet language in five years. But I am not entirely convinced, are you? 

(Thank you, Alvin, for drawing my attention to this. The image is from Gizdomo via. The Next Web.)