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1.
A person filed under "brother" or "sister" on a Facebook profile that has no real blood or legal relationship with the profile owner, but given a deep intimacy or friendship by a mutual agreemeent each of the parties gains this title. Its effects are limited to Facebook.

Derived from Facebook + sibling.

See also Facebrother, Facesister.
A:Hello Elizabeth, the other day i was reading your facebook site, and i was surprised to read you have *two* sisters, as your mother told me that she had only two daughters. Is she a step sister, or uterine sibling, or quarter sibling?
B:No Mr Harrison, none othe above, she is just my Facesibling, it's just a Facebook thing.
by keichix February 17, 2010