Friend

Growing up, I was always confused when someone asked me "Is he/she your friend?" about a classmate; I didn't know what defined someone as a friend. Eventually I got exhausted of the questions when I responded with "No" or "I don't know", so I discovered it was easier just to answer it with a trite "Yes".
To this day, I still don't know how to properly respond to that question; what is the line between an acquaintance and friend? There probably is no distinct line but this brings up the discussion of what makes a true friend? There's Facebook friends, where the majority of the people are just acquaintances and some you have only talked to with on one occasion. Facebook really diluted the emotional meaning of the word "friend"; Twitter and Google+ avoided this by using "Followers" and having the user label their connections respectively. I think Facebook did have something when they had limits on the number of friends; there is only a certain number of people you can be true friends with. While we would like to think that we could be the best of friends to a unlimited amount of people, people only have a certain amount of time that can be distributed to others.
Is a real friend someone that will always be there for you when you need them and you can ask them for any favors? A true friend will drive you to the airport whenever you need. Maybe it is someone that is fun to be around, easy to talk to, and sincerely cares about you. A real friend will know when something is off even if you don't say anything; they will even support you even if you don't listen to their advice. It is someone that thinks about you regularly even if there is distance between you two; each person makes time for each other (It's almost effortless). It's critical that the relationship works both ways, speaking and listening, giving and taking. Most importantly, the relationship is founded on genuine respect and honesty. Is the sign of a true friend is if they would take a bullet in order to save your life?
Of course there are levels of friendship between acquaintances and best friends. Those relationships are still very meaningful in different ways whether it is to watch or play sports, for career advice, etc. It is important to realize that you can't be best friends with everyone you meet but you can give your best effort to being the as good of a friend that you can be.
Let me know your thoughts/comment on the subject.