I don't feel it's wrong to have online friends. However, I get the feeling that the Internet makes people become less and less outgoing. Introverts will never learn how to make real friends if they always stay inside. For them, the Internet may work as an excuse "oh well, I could probably make friends in my town, but as I can still talk to people online, why should I bother and get outside?".
Sure it's more fulfilling to physically meet, and if it's a very strong friendship, two online friends may go out of the way to do just that. For a lot of people though, having this medium that they're familiar with and being able to talk to new people is plenty. If the person has a desire to be social and mingle, I don't think they would sit on the computer all day. And depending on the severity of introversion, physical social interaction could be less satisfying than an online friendship. I don't think the internet is hindering people, but opening more doorways for others. Ultimately those who know they should get out more, either will, or should buckle down and learn how to. The internet can be used as an excuse, yes, but so can a multitude of other things. Those who know they dislike the outside world can find comfort in just talking online, and I think it all comes down to the type of person you are.