A Facebook Favour is where where you do a favour for someone on facebook. E.g Liking some one's status. When Somebody asks does a facebook favour for you, they then owe you one back.
1)Will you do me a facebook favour and like my new profile picture

2)OK then
(person 2 likes person 1's photo)

1)Thanks mate, I owe you a facebook favour, anytime you want it, then just ask.

3 weeks later

2)you know that facebook favour you owe me

1)yes

2) well could i you pay it back to me buy liking my status

1)okay Dude
(person 1 likes person 2's status)

2)Thanks mate

1)I guess we're even now
by Cookie1994.uk January 15, 2010
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