You never know who you have shared your personal information with as you are always asked to create an account where you are required to fill your personal information to access online services. What happens with your information? Does that online service company take care of your privacy? Before sharing your privacy with any website, make sure it is legitimate so that your any information may not get leaked.
How to Use the Internet While Avoiding Hackers & Big Brother
If you are still on Yahoo! you need to reconsider the security breaches (it's no joke)... http://www.internet-security.ca/internet-security-news-archives-056/yahoo-found-negligent-internet-security-best-practices.html