They Aren't Supposed To, But...
If you are acting like an idiot on FB, stop acting like an idiot on FB. Seriously.
It doesn't have to be something someone finds on FB. If someone Googles you, FB posts and comments can appear without anyone even looking for you in FB.
Admissions officers are usually too busy to search for you online; however, if something in your application makes them wonder about you, they may indeed do a search. If they do a search, they are not supposed to use any information they find about you in their decision on your application. But that doesn't mean they won't use it.
The fact is, anything that you post on a social media site or send in an email is permanent and can be used in any way, shape, or form by anyone who wants to know more about you -- admissions officers, employers, the government, the law. If your FB posts make you look like a jerk, knock it off. It may very well come back to haunt you in some way, but you probably won't ever know that someone saw it and made a determination about you based on what they discovered.
In addition,the information you post can be misconstrued by someone who has no idea the context for your postings. Friends on FB are not necessarily your friends, especially those who might use against you the information they find there.
If you leave your laptop on when you're not in your room and your friends or siblings post to your page pretending to be you, it doesn't matter if you protest that it's not you (I know someone this happened to who was applying to grad schools.) The fact is that you didn't protect yourself and your future by guarding your identity, including your personal character and your political/social ideologies. That alone speaks volumes about your maturity and judgement, as well as your ability to be a positive addition to a community of any kind.
So, here's a good rule of thumb: If you don't want what you post on FB to show you in a negative light to future employers or college admissions departments, then only post comments, status updates, and photos that you would allow your grandparents to see.