For the under-20 set, they have NEVER known life without cellphones!!!! I remember, my first cellphone was a brick I got in 1995 when I graduated from college and was living in someone's spare room. I was a substitute teacher and needed a way for people to contact me at 5 am. But I digress...
I urge you to keep two things in mind.
#1: There is a person, with thoughts and feelings, on the other end of whatever you are about to type.
#2: If what you are about to type would be better said in person, with facial expressions and tone of voice, find a way to "be with" the person rather than writing it. Google Hangouts, Facetime, Skype, Oovoo... these free options offer you a way to see and hear the person you need to communicate with.
So much gets lost in translation on the web or via text. You can make or break any relationship by not communicating properly, by typing when you should be taking the time to talk.
This is true in social networks too. You can make or break someone's day in a forum or on Facebook, all with the power of your fingers. So use your powers wisely.
And for god's sake, don't break up with someone or give them bad news via words that you type. At least give them your face and your voice.