Rebecca Black -- I'm Not a Singer, I'm a BIKINI Model

There is a MAJOR Rebecca Black crisis on Facebook -- with allegations of deception, identity theft ... and most importantly a smoking hot bikini model ... who happens to be named Rebecca Black.

Turns out, there's more than one person on Earth named Rebecca Black -- one of whom is a self-described, "fitness bikini competitor." And she has the photos to prove it.

Black (bikini) tells TMZ ... sometime in the past week, her friend list on Facebook exploded from 800 friends to 73,000 ... and she had no idea how it happened. Black started getting emails from upset fans of the singer -- accusing her of intentionally hijacking the 14-year-old's page ... but Black (bikini) claims she never tried to steal anyone's identity.

Black (bikini) insists Facebook is to blame for somehow "merging" her account with Black's (singer) without her permission.

We called Facebook for comment -- so far no word back.