In that particular case some weeks ago, the girl noticed her mistake, closed the post and cancel her party. People found it "funny" to show up regardless. Her parents had to hire private security for their home, massive police presence to close the street and control the hundreds that came (even from far away).
Some further notes:
1) The police is opposing the "ban it" back-seat politicians, they say it is not feasible.
2) The boulevard press jumped on the bandwagon and accelerated events by persistent reporting (look at that, massive party) even after the cancellation. Afterwards they fueled it some more (already there are new parties on facebook, see here and here, we wonder how big these get). And that is the despicable part to me, not the poor girl that made the mistake and corrected it.