Online communities are social groups where the bulk of communication between members occurs on a computer network, and where it is possible to establish relationships of a personal nature using only that network.
Communities have varying degrees of openness. Every community is surrounded by fences of one kind or another, hoops you have to jump through to get in. Some communities have high fences; they are pretty exclusive. Others have low fences; they readily let people in. Open online communities are those which stand at the latter end of the spectrum. In such communities, people and discourse are easily accessible independently of geographical/economic factors or prior social connections. In what follows I'll simply speak of "online communities" although I'm really talking about open online communities.
Online communities and cultural mobility
Online communities are unlike previous forms of community because they dramatically alter the economics of motion across cultures.
First of all, entering an online community is extremely easy. You don't have to physically move to a particular location. You just need to click to have a look at a particular community. You can lurk for as long as you want and learn about the community without fear of embarassment. You can "try before you buy" and back up at any time. This is not true of other kinds of social groups (geographical communities, enterprises, professions, families, circles of friends...). Can you sample three companies, or three professions, in the same week just to see if you like their culture?
Second, the quantum of involvement is usually quite low, meaning that there is no specific minimal investment that you have to make to stay in. You can satisfy yourself with reading other members' contributions. You can post a few sentences and interact a little with members. Or you can get deeply involved in the community.
Third, and most importantly, online communities are not mutually exclusive. You can join as many online communities as you wish and decide how much you'll get involved in each on a day-to-day basis. You come and go as you please.
Because of this extra flexibility, it becomes easier to become a community straddler -- that is, someone who participates in several communities, be it simultaneously or sequentially, and who understands the culture of each to a certain extent.
Not all, but many people are willing to move across communities in such a fashion. Why? My guess is that these people are addicted to learning, and that moving is for them part of a process of personal growth. They've done their time in a particular community; they don't learn as much as they used to do in that group; there are questions they're asking themselves, directions that they find intriguing that aren't considered interesting in their community, but for which other communities seem to provide good leads. Moving around is a way of discovering their own likes and dislikes.
These people do not feel irrevocably bound to a particular community. They see themselves as multidimensional: as opposed to saying "I'm a doctor, don't expect me to teach you anything" or "I'm just a programmer, don't bug me with politics", they'll say "Well, right now I'm into this and that and that, and if you have something new to show me I just might take a plunge!"
Community straddlers keep the air fresh in a community; they help keep it alive. They contribute to the exchange of ideas between cultures which would otherwise hear little about each other. As an example, music evolves tremendously when artists dive into new styles and fuse what they already know with what they're discovering. The same can be said of science or other parts of culture.
The elevated numbers of straddlers among online community folks heightens the diversity and intellectual vitality of the communities they're involved in. This in turn helps those communities grow. As a community develops multiple faces, its contact surface expands: more and more people can get drawn into it because they see in it something that attracts them.
Thus a positive feedback loop is set up, where membership and diversity grow simultaneously.
From the Trenches
Content and conversation grows too, and over time online communities are increasingly becoming good places to be to keep in touch with culture -- not just online culture, but culture at large -- in an customized and interpersonal fashion. Online communities thus become increasingly attractive to the people -- artists, scientists, soul-searchers, activists -- who actually make culture. It is a way for them to stay up to date, get new ideas, and establish relationships with individuals that they find interesting.
More and more of these people realize that good personal contacts will come more easily if they narrate their own work, spread the word about what they're trying to find or achieve, and overtly link their own thoughts with others' thoughts.
This means that, increasingly, new culture --as a process, not as a product -- is being documented in real-time online by the people who make it. This is a significant departure from the way things have traditionally been working. Think of the number of journalists it would take to follow up on all those little-known participants!
Because they simultaneously fuel individuals' growth and cultural development, online communities will be embraced by culture-makers and evolve to become a major part of the process that is culture. It will happen simply because online communities give people more freedom to explore and cultivate their interests, and because they increase their ability to connect with like-minded people, as compared to previously available means.
Thus, to summarize, online communities will be embraced on a large scale because they better fit people's cultural aspirations.