In some cases, they can. They can also threaten your personal security if you don't make wise decisions about how you use them.
Like in “real life” you should use caution in your “virtual life.” You wouldn't give someone on the street a photograph of your child, your phone number, or your address, right? Well, believe it or not millions of people do that on the internet everyday.
On Facebook, it gives you a place to enter your employment information, your address, your telephone number, and your work history. If you choose to make your profile public, that means that anyone who looks at your profile can read all of that.
Facebook also has applications that require you to pay for certain things. (Usually things like “gifts” that you can send to people on your friend's list.) However, if someone hacks into your account they can steal your credit card information. This is particularly easy if you store your password on your computer.
In addition, sometime people hack into social utility sites, use a person's profile, and send out viruses to the people on their friend's list. When this happens, the victim's friends open the email thinking that it is from someone that they know, only to be hit with a virus that could crash their entire system.
To avoid this, make sure that you change your password on a regular basis and that you keep diligent watch of your activity. If you decide not to use your account anymore, have it deleted. Always ask permission before you include a photograph of someone on your site. Many people do not want their picture posted for various reasons. Don't add someone if you do not know them. It I very easy to pretend to be something that you're not when it comes to the internet.
Lastly, make your profile private so that only the people on your friend's list can see it. This will stop unwanted people from lurking on your site.