Internet use would only be a worry if it was getting in the way of us practicing some other life skill. If Facebook stopped people seeing their friends face to face that could have a harmful effect. But the evidence suggests this is not the case. If anything, people with more active internet lives have more active “meat-space” lives. Most of us are using the internet as a compliment to other ways of communicating, not as a substitute.
So there is no magic extra risk from the internet. Like TV before it, and reading before that, it gives us a way of practicing certain things. Practice will change our brains, just like any habit. The important thing is that we are part of this process, it is not just something that happens to us. You can decide how much time you want to put into finding pictures of funny cats, bantering on Facebook or fitting your thoughts into 140 characters. There will be no sudden damage done to your brain, or great surprises for your brain fitness. You would be a fool to think that the internet will provide all the exercise your brain needs, but you would also be a fool to pass up the opportunities it offers. And those pictures of funny cats.