TAN is an acronym for "Text A Nigga". Not a derogatory term, but a term of endearment used when trying to get in touch with a good friend or "nigga", if you will, sent via text or social network site of choice.
Jake: "What up?! Whatchu doin for NYE? TAN"
Isaac: "Word son! Let's go downtown! Glad you gotta hold of me nigga!"