I have seen it in quite a few chats on Facebook and Twitter.
"Sic," like so many things in academic English writing, is derived from a Latin phrase: sic erat scriptum, which translates to "thus was it written." It's typically used when quoting someone to denote a grammatical or other error in the quoted text, to specify that you did not make that error yourself as a writer. I'm surprised you've seen it so often on Facebook and Twitter; usually, the convenience of web-based discussion clearly delineates quotes made by others.
The word sic is probably used as an adverb here -
sic (adv) - Intentionally so written (used after a printed word or phrase).
You may observe this on chat and social media in the context wherein the opposite person writes it intentionally though knowing that the word is misspelled.
When it is used in brackets it means so or thus to show that an odd or something doubtful reading is what was actually written or printed.
Example from OALD:
In the letter to parents it said: ‘The school is proud of it's [sic] record of excellence’.
Here, the word it's is incorrect but copied from the original document. You know that this is incorrect but still will have to write. To show that you are aware of this mistake, you put (sic) after a wrongly spelled word.