As a high schooler, it has become a mandatory part of the social experience. I can count on one hand the people in my grade without a Facebook, and many real life conversations will start with "so did you see X on Facebook last night?" Although its for the most part a large waste of time and I don't really want one, I use it because I would like to have a social life. Its sad, however, that many of my friends fail to see how superficial it is.
I wouldn't say that it's seen as a replacement, but I definitely think it's become and integral part of the social life. For example, a lot of the time if someone is hanging out with another person, they will make a status announcing it. There's really no real reason for this, other than to boost social currency by saying "I was with Person X." I don't think its really replacing hanging out and doing stuff with people, but its making it so much more real-time, since all of your friends know what you're doing literally as you're doing it.
However, one thing that Facebook is good for is coordinating groups. I'm in a class council group as well as groups for a few AP classes. Since everyone is always on Facebook, it makes setting up and coordinating things while getting feedback super easy. For example, a couple days ago, someone posted after school in the AP Stats group about getting together for a study session. A few hours later, around seven of us got together at Barnes and Nobles.
