The existence of online communities can be explained by the human kind's need to communicate. As the famous quote from John Donne goes, No man is an island. People spend so much time online, on the web that they start building a community there, sharing conversations with people having common interests and hobbies. And as the web connects people from all countries and cultures, this permits people to learn and share experiences from all around the globe. It is not only limited to one's physical neighbourhood or group of friends. During the past decades online communities have tremendously developed and have become part of our socio-cultural reality for some of us, at least. For instance, I have an offline life which is very much complementary with my online life. Often at dinners at relatives' place, I'm asked, "so how is your blog doing?" or when I try to take a picture of someone, I get the following response: "I hope this is not gonna be on your blog!" Ten years ago I don't think anyone would have known what's a blog or given such an importance to his/her online image.
Another thing is, we are spending more and more time online. The internet is no longer used just for the sake of checking ones mail or finding informations, it is used for more personal reasons from finding dates, making friends to earning money and publishing ones thoughts. For these different purposes exist different sites like myspace, hi5 and blog hosts. A common point in all these sites is the interaction that they provide to their members.
In brief, I attribute the existence of online communities to mankind's irrepressible need to communicate and share thoughts irrespective of where they are. In other words, we are building a parallel world, an online world.
p.s: This post was written cause I'm participating in MBB's Theme Of the Week.