According to this piece [found on Social Media Today], more and more students are failing English. A big factor in this trend is the freeform writing style of social networks like Twitter and Facebook, which is changing the way people communicate. At least, that's the claim of a piece in the Canadian Press. According to the article "(at) Simon Fraser University in British Columbia, one in 10 new students are not qualified to take the mandatory writing courses required for graduation." And academics are, in part, blaming social networking.
As far as I know, there is no thorough similar research on Dutch, but I cannot help but feel that attention to and care for spelling is... well, not improving to say the least. However, I am not certain that social networking is to blame. If anything, social media have created the first generation in a long time who express thoughts in a written fashion on a massive scale. The use of emoticons and abbreviations obviously does not help in improving one's spelling. But on the other hand: my gut feeling tells me that students' knowledge of spelling and grammar is already poor to start with.
If we want to change this ongoing trend, we should rethink the current curricula in primary and secondary schools where the apparent fear of spelling exercises, dictations and boring grammar periods has replaced the insight that they are essential building blocks for any kind of language acquisition...
Note: an interesting piece on how texting is actually improving spelling can be found here.