They are beginning to do studies on this and, as is the case in most studies, so far the results appear to be conflicting and contradictory. Whatever the outcome of these studies, in my opinion the positive aspects far outweigh the negative aspects. It can be addictive, but so are a lot of things and an addictive personality will always find something to be addicted to whether it be the internet, television, sports or even work - to name a few "positive" addictions. It can be said that it is a solitary pursuit, but so is reading and more so. I love to read and you can usually find me with my nose buried in a book. And although I said a solitary pursuit you can connect with other people, if you so choose. I still find it fascinating that you can connect so easily with people from all over the world.
The internet has so much to offer - whatever knowledge you seek is easily accessible - it's a great educational tool and an important and indispensable tool in the workplace. It brings communication to a new level, whether it's just an email to a loved one or friend, or participating in a chat, or joining a group of people with similar interests as your own and sharing your opinions or concerns, or just having fun. It can even be therapeutic because in such an anonymous environment there is more freedom of expression - just like sometimes telling your problems to a stranger can be more beneficial to you than telling a friend - the fears that usually inhibit you, whatever they may be - fear of judgment, fear of rejection, vulnerability, etc., are gone. In this sense, anonymity does have it's appeal.
Of course the internet like any other medium can and will be abused, that's just the nature of the beast we call the human race. But when all is said and done, in my humble opinion, I think the pros far outweigh the cons and if the internet does contribute to the social isolation of people it does so to a minor degree and for the most part to a positive effect.