Somebody's, is singular, the 's marking possesive, so "Somebody's ass", being the ass belonging to somebody.
Somebodies is simply a plural noun. The way we see family > families.
Somebodies in this case would mean like, 'oh, your a nobody, those guys, they're somebodies". As in they're people of note, they're famous.
This has been the Grammar Minute.
Oh the irony