When hi5 was launched, it had a reduced number of members. Over the years, it became more and more popular, so there were many people interested in finding out email addresses of the people which were having profiles on hi5. When you sign up, you have to give hi5 an email address which has to be verified after, address with which you're going to login every time you use hi5. So here started everything.
So, if you know someone's hi5 profile, you can paste the link of his/her profile into a website, which will return you the email address of the person whose email you want to find. And this happens without his/her permission. First websites of this type were created long time ago, and they are still working pretty well.
Hi5, which is "the authority" which should have done something in this situation long time ago, didn't move a finder to change something until now. It simply let the privacy of the users to be violated. Over the time, it has been shown that hi5 very vulnerable against many attacks/exploits, so, what's worse, an email address of somebody can be found even if that person has his/her profile private!
An example of a website which can do what I told you is yahoo-invisible.us (this one can find only profiles which were registered with Yahoo! emails). Personally, I don't support this type of websites, but I just wanted to show you how easy is to violate the privacy of millions of users.
Will you be still using hi5 from now? Do you agree to have your privacy violated this easy?