There are great variations in language and communication in discussion groups on the net, just as is the case offline as well. The variations are contextual, meaning that if the writer wants to chat, the written language might vary from the one used when (s)he for example searches for information. Therefore it's difficult to draw any general conclusions such as a presumed, ongoing decline in the quality and correct use of written Swedish, says a researcher colleague in Åbo, Lotta Collin.
She's not however worried about the written language deteriorating due to discussion groups on the web as she believes that people can tell apart "web language" and more formal language. She defends her doctoral thesis, where she linguistically analyzed 1000 Swedish postings in a discussion forum for diabetes, tomorrow in Åbo.
There's an article in Swedish about her here and here.