A teenage girl who included her address in an invitation to her birthday party on Facebook was alarmed when 21,000 people confirmed their attendance.
The bash was called off when the school pupil was swamped by RSVPs from unwanted would-be guests on the social networking site.
The girl had reportedly only intended to invite 15 friends to her 15th birthday party and did not mean to make her address public.
Hertfordshire Police said officers would be patrolling the area in Harpenden on October 7 - the date it was planned for - in case large numbers turned up anyway.
Harpenden Neighbourhood Sergeant Lewis Ducket said: "We are aware of this and have been assured that the event is no longer taking place.
"I would urge people who may be planning to come to Harpenden for the party to make other plans.
"We will have officers on patrol in the area on October 7 to provide a reassuring presence and who will be able to deal with any issues, should they arise."
Although the original invitation was removed from the site, a new version was visible to all Facebook users today.
The event was not created by the girl herself but her address was again posted on the site for all to see and 19 people had confirmed their attendance.
Other linked events were also still visible to all users of the site, including a "cleaning up party" the following day, which had 80 confirmed guests.
The incident follows criticism of Facebook's privacy settings, with concerns being raised that they are too complicated.
A spokeswoman stressed that the privacy settings available for events created on the site are separate to those for users' profiles, on which they post information about themselves.
When someone creates an event on Facebook they can tick a box saying "anyone can view and RSVP (public event)".
If this is ticked then it is a public event, meaning anyone can view the content and respond.
The spokeswoman added: "If users do see content on Facebook that they feel is inappropriate or unsuitable, we have clear reporting links on every page, including event pages, for users to flag it.
"We also provide people with the tools to manage their own content so with events, for example, there are clear tools to allow you to control who can see and respond to the event.
"Helping children to use the internet safely must be a partnership between service providers, parents and teachers. We have a lot of resources on our safety centre...to help people use Facebook responsibly and we encourage people to use these."Reuse content