Fair. But I've also witnessed this outside of work environments. People are so strongly (and as I say, this isn't bad on its own) tied to past relationships (HS, college) and easily connected to them via FB, Twitter, etc. that they aren't making relationships in their new locations when they move. The latter part is the problem. You're in a new environment. You're working and living with new people. You need to connect to them in some manner, if you remain strangers with those around you (work, neighborhood, etc.), then you will be lonely and unhappily disconnected from people unless you're particularly introverted.

I missed your parenthetical earlier.

> you're still in touch with old friends, and whenever they move close enough you know it and can rekindle the friendship

This is a big if. I'm speaking of people who have moved across the country or state, and where the likelihood of this is near zero. The only way they'll rekindle the friendship (specifically being physically local) is if they move back themselves, rather than waiting for their friends to move to them.

Well, I'm in the Bay Area and grew up in LA, so the drift is very prounounced. People at my old workplace who came from farther afield (e.g. Canada or the East Coast) tended to take a lot more initiative in socializing at work.

In social situations, they are the same thing.