The concerns with virtual communities are just and hold true in most situations. But as far as people who really need to communicate with anyone about very serious subjects, like their sick child, well they just really need to "talk." Luckily the Internet is an outlet for them to express what they truly need to say, but are not ok saying in person to someone they know.
The Internet provides a unique sense of identity security that doesn't exist in the real world with the exception of confessional conversations where a person talks behind a screen. But the biggest danger that is apparent in both a virtual community like the Leukemia discussions, or a simulated world like Habitat or the SIMs is the danger of trust. As hard as you may try to trust someone you've never met, is it truly possible?
Can you fully trust the advice or actions of someone you've never met?
I feel that in some cases you can, but in most you cannot. Or at least if you take their advice, and you meet them later you will feel better about your decision to trust them if you enjoyed them in person. There is a unique interaction that occurs when people interact and although it is by 'games' like Habitat and SIMs and modified physically by chat rooms, Trust is one aspect of life that does not transfer from the physical world to the virtual world. Trust is gained only in the day-to-day, face-to-face interaction people make everyday. Even though your security on the Internet and in game is constantly reassured, you never completely trust the source.
One random example, every time you buy something online, the thought that someone my find your credit card online still exists. The trust is just not completely there. There are the necessary precautions you take to protect yourself such as firewalls etc. But if they do get your number and use your card, it was all done behind your back with you having no idea that they are stealing from you. Then you realize you really cannot trust anyone online because you don't know him or her as a person.
But in real life, if your credit card is stolen from you, you are part of the problem, you left your purse open and they snuck it out, or you dropped it and they picked it up. It happens right there in front of your face and it is your fault. We don't trust all the people around us and that is why we protect our belongings, but we do trust people that we know and are close to us. We'd let them watch our things and give us advice because we trust them as a person.
I really believe that no matter the security, you cannot completely trust anyone you don't know on the Internet.