[archon]

I don't think it ever fell out of use, it just isn't written.

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.

[frosted-flakes]

That's what I meant, in writing. Of course, it's still spoken that way.

[mywittyname]

There was a push in American education against using contractions in writing sometime in the earlier 20th century (perhaps earlier). The APA style guide says to avoid contractions in all official writing. People probably continued to write as they were taught in grammar school.

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."