The humanitarian developers behind World of Warcraft have also discovered a way to bribe gamers into turning off their computers and going outside. If you log off for a few days, your character will be more 'rested' when you resume playing, a mode that temporarily speeds up your leveling.
The only reason we don't notice how absolutely interwoven our thinking processes have become with older technologies - pencils, paper, electric light, penicillin, fire - is that they're old, so we've ceased to notice their effects.
I don't think the Internet has replaced cities in any significant way, nor really could it. Cities are dynamic - and deeply seductive for the people who flock there - because they broker all sorts of fantastic and useful connections, cultural and economic and social.
More than any other modern tool, computers are a total mystery to their users. Most people never open them up to fix them or to see how they work.
Transactive memory works best when you have a sense of how your partners' minds work - where they're strong, where they're weak, where their biases lie. I can judge that for people close to me. But it's harder with digital tools, particularly search engines.