[blub]

Not just American expat, but any kind of expat (or if you prefer foreigner) because these are the people most likely to become new acquaintances and friends since they're likely also looking to make new friends. Locals will always be more reluctant and less interested to make friends because they already have a social circle and it takes effort on their side.

University towns only really make a difference if the hypothetical person moving there is college age and if there's a significant number of foreign students. Otherwise why would German college kids want to socialize with a late 20s foreigner?

Someone which moves to a new country will leave their entire social circle behind: family, friends, personal and professional acquaintances. It's an immense loss that will never really be recovered and the older you are the harder it is.