seriously? Have you read any news of late? Yahoo MoDaCo any of that mean anything to you?
When you register to any site with any details, over time you will probably provide some VERY private details, real name, real address, email address, equipment you own, etc etc
All of that can become available to the hackers when they 'guess' your simple password, they then glean the fact that you have used the same username and passwords on multiple sites and you can bet one of them will be the one that provides them with the details they need, either financial or social, and then you are compromised.
Due to the way people use social networks today, the lack of thought as to how and why that information is stored is stunning.
Don't think this is true? Then check if you have ever provided any of the following to a 'non serious' site
Real name
Real Address
Date of Birth
Name of family members, animals, schools
Details of purchases, with the type of cards used
Holiday locations and times
Travel details
Food preferences
With VERY little effort, you can build the profile of people using a combination of the above and then steal / impersonate / defraud
If you have issues with passwords, then consider using a quality password manager or check out this page for help Password Haystacks
Some of the easiest ways to socially hack someone is to watch when they enter a password and due to 'reasons' get it wrong so many times that they either say it as they type it or type it so slow that it so easy to see..
And YES the hack part of my username is for a reason