I don't want to speak for ArkyBeagle but I think, when young adults move, they often move for work - so you show up in a city where what you're there to do is work, the people you know are through work, you don't have the balanced social structure of family, friends, and work, so you don't end up spending healthy time relaxing with family and friends, you just... work. That's my guess.

That is better than I could have summarized it. I would say that for children, it may be even harder.

Then again, I moved a child from what seemed a very good but isolated (for her) situation and she developed much more of a network after the move.

I am echoing Peter Whybrow from the BookTV presentation of his book "The Well-Tuned Brain".

Moving disrupts your (organic, on the ground) social network.