I'm from the US south, I use both "them" and "'em" in everyday speech depending on context, but I never write "'em", even where I would use it in speech. This is partially because "'em" in written form ends up conveying a slightly different tone or meaning than I might intend. "Them" is just a word; "'em" has more nuance and is more easily misunderstood.
If it were to make a comeback today, it would probably be in the form of "m". Like, "ru goin w/m?"; or "m ain't spiders. mr2 spiders."
“Go get ‘em boy”
I grepped a bunch of source code I have lying around, and I found it used in OpenSSL (3 times), the Postgres C driver, and the Snappy compressor -- the latter because it includes a complete copy of ALICE'S ADVENTURES IN WONDERLAND!