Posting like crazy about getting hammered doesn't mean you're schwastey at work.
I don't even think they're supposed to ask you why you're calling out. I always just say I can't/won't make it and they say "okay" and that's that. Even if I was sick, who cares? They're not gonna fill out the sick hour paperwork for me. I've never actually used sick hours, but I interpret them as mental health days... Which means I might just be sitting around watching netflix with firefly sweet tea vodka and such avoiding target... For my mental health.
If the fb post had nothing to do with work and wasn't a threat or something crude about another tm, I'd call the integrity hotline. You're being super vague, though so I can't tell if the fb post included something that would warrant firing or not.
I looked into past lawsuits about social media related firings a while back, and it depends on a few things whether or not it's legal... And it's sort of a new thing so there aren't many precedents set