Why think about it? For those that take part in and enjoy the OnlineCommunity, this may not be an interesting question. But for those that build or promote the OnlineCommunity, it may be a vital question to understand what a community is (without reducing this to a fight about definitions) and how it may be perceived from outside. Because those that study real world (offline) communities - e. g. social scientists - our OnlineCommunity(s) will not qualify as communities, because you can hardly find the typical qualities of a RealWorld community in an OnlineCommunity.
Communities with online presence appear to have more chances for survival/developement. A strong OnlineCommunity component might be an additional advantage. A guide to IntentionalCommunity?(s) can be found at http://www.ic.org/ . Somebody should tell them about Wiki culture..
A community is much more than the sum of its parts. The strength and vitality of a community lies not in the individual CommunityMembers, but rather the PersonalRelationships within and between communities (see WhatIsSynergy).
OnlineCommunities are like ongoing discussions with predefined protocols for social etiquette, such as where to meet, what to talk about, and also how to talk about it.