Did I commit a Facebook faux pas?

I have an old high school friend who I've only seen a few times since high school. She lives about 30 minutes away, we have toddlers the same age, and we are friends on Facebook. This friend recently posted as her status update a public invitation to an event at her house. It was along the lines of, "Hey, if anybody is not doing anything for New Year's, I'm having a potluck at my house. Call me if you want to come."

My husband and I didn't have plans, so I private messaged her to see if we could come and ask what we could bring. She has seen the message but not responded.

Was I wrong to assume we might be included in this public invitation? Should I have assumed she was only intending to speak to people who she has seen more recently (or people who have her phone number)? How, in general, should one respond to public Facebook invitations from people with whom one does not have lots of in-person interaction?