Selena Gomez BANNED from Facebook

Selena Gomez has been banned from Facebook until further notice -- but here's the thing ... it's not the famous Selena Gomez ... it's a girl in New Mexico with the same name ... and she's PISSED.

18-year-old Selena MIRANDA Gomez tells TMZ, she tried to sign in to her Facebook account on Wednesday ... but was denied, receiving a message instead that her account had been disabled for being "inauthentic."

According to the error message, the account was disabled for violating Facebook's terms -- which state, "Impersonating anyone or anything is not allowed."

But clearly, the non-famous Selena isn't impersonating anyone -- and now she's really upset that she's been unfairly banned from her social networking profile, telling TMZ, "I AM NOT AN IMPOSTOR ... My name is not hers on my page. In fact, I even put my middle name on my FB to clear up any confusion."

She says, "I did not have one single famous friend. I did not refer to myself as [the famous Selena], and I did not have any pictures of her on my page!"

Selena says she's reached out to FB to reactivate her account -- but so far, no luck.