Tagged.com is an actual social networking site, and it's okay to click on it and join. However, you must give up your email address and your email password. Then, Tagged.com takes all your email addresses of all the people you've ever written to, and sends them an invitation. I never give up any passwords to a third party site, and neither should you!
It's not a complete scam, but their invasive processes have invited many complaints.
Read the entire story, here.