But without a formal invitation how can you just show up? I doubt the formal invitation would require facebook.
If so, if someone wants to invite me, I'm sure they can find a way around Mark Zuckerberg to do this. If not, their loss. I'm not a party person though, that might have something to do with it as well. (I do get around to my share of hacker conferences, but they somehow don't seem to use Facebook...)