A girl who claims to be lesbian, but really is not. This is claim is usually made due to several factors, sometimes all combined. Those being, Confusion, Aiming for acceptance from actual lesbians or other facebook lesbians, sometimes these girls have never fully matured and been able to open up and be comfortable around guys, so they just settle for girls who they are more comfortable with, especially with showing their bodies off.
Essentially a Poser.
Dude 1: Dint that chick just suck your dick last week? facebook says shes a lesbian..
Dude 2: Shes just a facebook lesbian man, shes a trendy homosexual.
A female on Facebook who lists herself as being in a relationship with another female friend, while in reality she is heterosexual and lists the reltionship out of some bizzare feminine vanity.
Dude #1: "Oh shit son this chick is into chicks!"
Dude #2: "Nah dude, she straight, shes just a Facebook Lesbian. See? Her preference is men."