That's actually a very good point. The way I see it, although I may be wrong, is that people that have difficulties in reading/writing are a minority, at least in developed countries. Thus, they are nevertheless exposed to a way of speaking that is mainly influenced by the majority, who are influenced somehow by the written word.

On the other hand, what strikes me as a contribution of Internet to the language is our capacity to communicate through videos and pictures. Which means that our vocabulary is narrower. I don't have to be so elaborate when describing my room; I can just take a picture, and tweet it.