Usage is different from grammar.
Grammar operates according to rules. Usage often breaks the rules. Usage is the way in which words and phrases are actually used by a community of speakers.
Some usage is only heard in conversation among friends. Other usage is suitable for formal speech and writing.
Nobody speaks the same way in every situation. Even the Standard American English taught in school is spoken in different registers.
A register is a way of speaking. For example, all of the following situations call for different registers of English, ranging from informal to formal:
a family get-together
a school lesson
a conversation with close friends
a conversation with acquaintances
a job interview
a college lecture
an exchange between a merchant and a customer
a policy discussion in a business meeting
a friendly letter
a newspaper article
a blog on a serious subject
a blog intended to be humorous
a mystery novel
a technical manual
a book about insects
Many speakers don’t seem to know that language acceptable in private conversation may not be appropriate in a public forum like Facebook.
The Post Category “Basic Usage” includes articles about grammar as well as usage.