Top Definition
When organising an event on Facebook you can use the rule of thirds to estimate how many people who have said they will or may attend will actually attend.

You can assume that approximately two thirds of those who have said they will attend will turn up, and one third of those who have clicked maybe will turn up.
Bree: Wow, we've got 69 attending and 93 maybes! We better buy some more booze!

Alex: Hold on, by the rule of thirds that's only 77 people, so we're okay with what we've got.
by boykitten May 09, 2011

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