I'm sure you've thought of this already, but in my mind, a city/polis/whatever is defined not by its residents, but by its power structure. If the online community has a defined ruling mechanism, a defined enforcing mechanism, and a defined set of rules, then it may be viewed as an equivalent to a physical city.

[...the other question, of course, is that with some obvious exceptions, a city is a subdivision of a country. So, an online community 'city' can only be valid if it is in fact one of a group of similar entities that are united by a common hierarchy. Just to throw out a hypothetical - would you have considered AOL chatrooms 'cities']