Wow ... you are one of the few people who actually just opt to not attend events you don't feel obligated to go to rather than b!tch and moan about how "tacky" and "gift-grabby" it is to do something like this. Good for you!
FB made the whole world lazy.
While I agree with the bolded, I have to say that when someone commits a breech of etiquette, I always assume that they simply don't know any better (the not knowing any better here being that showers are usually semi- intimate events, and the way that guests are invited should reflect that as opposed to a FB cattle call).
I try not to assume that the reason for the breech isn't coming from a bad place (being "gift grabby"), but simply ignorance of the basic rules of etiquette. OP, I realize that you do not know the MTB intimately, but if this were a close friend, would you snub her?
Also, try to keep in mind that the MTB might not even realize that her hostess has chosen to invite people this way. It should not be a reflection on the MTB, but of the hostess.