someone asked

Do you find Facebook to be a safe network for people to use?
Yes.

Do you know if it is very difficult to hack into Facebook?
No, I don't.
I will assume it has fair security since I don't see too much on the news regarding its security and they are still pretty popular.

I have seen different situations where spam has been sent to me or my friends. How can this be avoided?
I would suggest having two email accounts. Use one for registering for websites, etc, call it your junk email account. Use the other for friends, call it your real email address. The idea here is that if you are registering an account with a website and need to confirm that you are using a valid email address, you can, without giving them the address you use for friends.

Is there a way for hackers to find out your personal information without being your friend?
It depends on how good of a hacker they are and how much personal information you put online.

What advice would you give teens who are using Facebook, in order to keep themselves safer on the Web?
Safe from what? Let's assume you mean physically safe. First you have to ask why would someone want to attack you? Two reasons come to mind, they either want something from you or they are angry with you.
So in these cases the best ways to stay safe are to:
Not let people know you have anything they want
Not to make people angry
It's worth asking yourself questions about these things:
What kind of things might people want that you have? How might they find out that you have them?
What makes people angry? How might you provoke someone?

Now avoid doing any of the things that you listed. Most of the people we socialize with online know who we are and can probably find out where we live. But if they are strangers, then you have a good chance of making it impossible for them to find you by not associating your location with ANY of your online social networks (facebook, twitter, blogs, etc). Some people are clever enough to look for your username on another network to see if they can find more information about you.

A few tips:
I would (as a teen) avoid putting pictures of myself online.

Never ever (even as an adult) say bad things about other people online. Saying bad things about people almost always makes you look like a jerk...and makes other people angry with you (including your parents).

Don't reveal personal information through your username (ie your age, year you were born, sex, location, city, state, school, real name).

Don't spend too much time socializing online.

I just wrote down my thoughts. I'm sure others have put more time and energy into this so if you're serious about being safe, you should probably do some additional research. http://lmgtfy.com/?q=online+safety+for+teens