Could you just ask the MTB for the hostess' info?
It doesn't sound like the shower is a surprise, so I don't think it would be a big deal to ask. Messaging on fb seems a little creepy to me-- not sure why-- probably because I am old
I would ask my MTB friend for a contact phone number or even an email address and say/ type something like this to the hostess:
"Hi! I am a friend of MTB, and I know there wasn't any RSVP info on the invite, but I just wanted to let you know that I will absolutely be at the shower in case you need a final head count. See you there!"