While the population has always been mobile - in part, highly mobile - day-to-day community tends to be extremely localised. [This applies equally to geographically-distanced virtual communities: bloggers put permanent links to other blogs they read regularly on their sidebar, but, how many do they visit - as opposed to monitor through RSS - on a daily basis? In my case, only three or four. That's localised.]
Historically in English towns and cities, traders arriving from the countryside or the Continent set up localised communities. We can trace these through street names, such as Threadneedle Street, where seamstresses and tailors lived and plied their trade. The pattern was later echoed by Quaker entrepreneurs building model neighbourhoods for their employees, such as Bourneville in Birmingham - and even by the contemporary trend in city planning to designate Quarters, such as the Jewellery Quarter, also in Birmingham. This concentration meant that anyone arriving from outside the city with a particular trade had an immediate starting-point for employment and relationship with like-minded people. They did not set up ghettos, in the sense of self-imposed segregation to maintain the 'purity' of their community. They didn't form one big monopolised workplace either, but clustered a trade together, regulated by Guilds (apparently the advantages of such clustering outweighed the potential problems of too much direct competition).
Broadly speaking, churches - and pubs - took a different approach, spreading through the city rather than concentrating their resources in one area - long before, say, schools did the same. (Though the Quaker entrepreneurs provide an interesting alternative model.) And that is valid, of course. But the rub lies where mobility and locality collide: as a friend said to me not long back, [something along the lines of] "We spent two years getting to know the neighbours, building relationships; and then a couple of the families who had been most open to us moved out in quick succession...it was like a kick in the teeth."