This is a multi-faceted subject. You might begin with some of the lack of formality in texting--as well as the abbreviations like LOL, BFF or any of the host of others. Perhaps you want to discuss who it is appropriate to be informal with and where is it taboo. You might also deal with dialect differences. Where I live (in southern Ohio), it is very common for folks to leave the "g" of an -ing ending (ie. Going becomes goin'-walking becomes walkin')- yet it is important for people to practice standard English in a formal or even a semi-formal setting.
Other social contexts such as casual friendships are dependent on the relative age and age differences of those involved. There is also a general difference in communications between the sexes.
I have posted a link to Purdue University's writing site with a specific link to formal language. I use this site often and I think you will find it helpful.
'Formality' in language is very close to other forms of behaviour(non-linguistic). It implies:
1. Extreme preparedness;
2. Consciousness of approach;
3. Selectiveness of manner;
4. Studied logicality;
5. Strong purposiveness.
Whenever you are using language formally, you are very careful about your vocabulary, trying to avoid all sorts of slang and colloquial expressions. You will generally abide by all the sanctities of grammar, the tit-bits of construction, the purity of the idiomatic. Your sentences will be logically connected with a systematic build-up. You will maintain all possible care to sound solemn, decent, rational and decorous.
Think the manner in which you chat with your friends; you are always funny, spontaneous, ungrammatical, moody, colloquial, showing a deliberate insolence with your vocabulary, style and tone. Now think how things change when you are talking to your teacher, or the priest, or your lawyer. Had you ever been at a court of law as the judge delivers his verdict, you could have felt what formality in language might be.
A large part of our social behaviour is linguistic, sometimes formal, and at other times informal. Informal is casual, often audacious and even shocking. Formal language implies obedience to social/behaviorial norms: occasion, subject, purpose, relationship between the speaker and the hearer etc. guiding the form of language.
Check out these websites for more information: