“I'm trying to break my Facebook addiction, so I haven't been on in a while.”
A friend asks you if you've seen a funny photo that a friend posted on Facebook. You used to use Facebook too much, so you've stopped using it. You haven't looked at it in 10 days, so you say this.
I'm trying to break my Facebook addiction, so I haven't been on in a while.
You use "I'm trying to ___" to describe something that you want to do, but isn't easy.
I try not go visit Facebook too often.
This means that, in general, you don't go to Facebook.com – maybe because you think it's a waste of time. On the other hand:
I'm trying not to go on Facebook too much.
This sounds like you want to stop going to the site, but it's difficult for you to stop.
When you're "addicted" to something, like drugs, it means that you have to have more and more of it, and you can't stop yourself. You call this problem "an addiction". But people also use the word "addiction" to talk about less serious things that they can't get enough of, such as:
I'm addicted to "24", so I don't know what I'm going to do now that it's over.
To "break an addiction" means to stop yourself from being addicted.
When you've "been on" a website, it means that you've visited it. The word "visit" sounds a little to formal for friendly conversation, though. So in casual conversation, people ask:
Have you been on People.com recently?
One other note: you use "been on" for large websites with a lot of functionality, like Facebook, Twitter, eBay, etc. For smaller websites that don't do much (like PhraseMix.com), you should use "been to" instead.
"A while" is some length of time. It's not clear how long "a while" is, but you can think of it as a little bit less than "a long time". In the example above, "a while" might mean a few weeks or a couple months.
You use "a while" in spoken English, but not often in writing.