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Over the past several years, the web has been kind to extroverts. Social networks have offered a new platform for people to broadcast their thoughts, connect with each other and expand their contacts in the online realm. The social web has even ushered in a new kind of extroversion, in which people who might be shy or uneasy in traditional social settings can express themselves online.

Much less noticeable is another trend: the rise of the web introvert. But while some web introverts might be introverted in the classic sense — that is, uncomfortable in social settings — many of them aren’t shy at all. They are simply averse to having a public presence on the web. And in time, they are going to present a problem for social sites like Facebook and Twitter, whose potential growth will be limited unless they can successfully court them.

Web introversion isn’t a question of technophobia or security concerns. Anyone who has tried to build out their online networks on Facebook knows that there are a lot of people they know in real life that they can’t friend online. Many people who have been involved in technology for years — or who are entirely comfortable shopping at Amazon (s amzn), paying bills online, buying songs from iTunes (s aapl) — will have nothing to do with social networks. Others see it as a chore necessary for their jobs. Still others have accounts languishing on all the major social networks.

If you ask a web introvert why he or she isn’t into social networks, the response often comes down to a matter of trust – or rather, a lack of it. It’s frustrating enough that each social network has its own etiquette to master, but many people are loathe to make the effort because of the unpleasant reality that there is no such thing as privacy on the web.

And typically, the more that web introverts understand the nature of the web, the less willing they are to expose themselves on it. For while you might start off thinking you own your tweets, you really don’t. And if you don’t want your Facebook information open to the public, you need to follow closely that site’s constant privacy changes. Moreover, regardless of the site, a casual comment that, in an offline conversation would be forgotten, is preserved for years on the web — and could come back to haunt you.

For extroverts, this is all just part of navigating the social web. But enough people are uncomfortable with social networks that it’s going to become a barrier to growth in the coming years. For now, Facebook’s growth is continuing simply because there are more and more extroverts signing up. And Twitter is still in the stage of experimenting with ways to make money.

Eventually growth rates will slow and these companies will see web introverts as an alienated part of the market that they need to court. Each introvert, after all, is a lost opportunity for revenue. But it may be that these characteristics are so inherent in the social web that such people simply can’t be courted.

Image courtesy of Flickr user creatingkoan.