Whether you're a constant tweeter or just check Facebook to see pictures of the grandkids, know this: Nothing you post online is private. So here are the top four things you should NEVER reveal on any social networking site, courtesy of Yahoo Finance: * Your full name and birth date. There's nothing wrong with putting the day you were born but never add the year. A study by Carnegie Mellon University found that with your birth day, year and name anyone can predict most or all of your social security number. If the bad guys get that, they have the key to stealing your entire financial life. * Your vacation plans. You might as well post "Rob me, please," on your page, because that's what criminals see when you post how excited you are to spend the next two weeks in Maui. Go ahead and post pictures when you get back but keep your plans private or risk a home invasion. * Which brings us to the third piece of information to keep private: your home address. A recent study found that 40 percent of people list their home address on social networking sites. * Also, a lot of people unwittingly leave password clues on their pages. Banks and utility companies often ask security questions like your mother's maiden name or your favorite song. If any of this info is on your page, you might be telling identity thieves exactly what they need to know to break into your account. Finally - be careful about posting what you really think on Facebook. Employers do check social networking sites to determine who to hire - and who to fire. Need proof? In just the past few weeks, an emergency dispatcher was fired in Wisconsin for revealing drug use, a waitress got canned for complaining about customers, and the Pittsburgh Pirate's mascot was dumped for bashing the team on Facebook. One study done last year estimated that 8% of companies fired someone for "misuse" of social media. Consider yourself warned.