Given that "birthday" is, apart from obviously being the actual day you were born, an "anniversary" of the day you were born, there's no definition or rule that says it is only something that happens annually with many "celebrating" the anniversary of their child's birth monthly up until the child's first birthday.
It's not just a US thing, seems to be common throughout the "Western" world for some reason.
Unfortunately two of her friends have done so over the last few years, with their facebook being full of such nonsense. It's one of the "joys" of having a younger woman as, naturally, her friends are younger as well and are truly of Generation Social Media where the one of the only aspects of their life which does not get mentioned is the size of their morning dump.
You soon learn just to skim over the crap, as bludgeoning them to death with a shovel is frowned upon here.