The suspect, Akim Jackson of Oakland, uses the website myredbook.com to contact his potential victims, police said.
Investigators said they believe there may be additional victims. Anyone who was a victim or may have more information about Jackson should call the department's vice and child exploitation unit at (510) 238-3641.
Jackson is a black man with a light complexion and is 6 feet tall and weights about 220 pounds, police said. He is known to drive a red 1999 Ford Mustang with a personalized license plate of SASSSYE.
Jackson was arrested in Las Vegas in late July and is now being held without bail at the Santa Rita Jail in Dublin.
He is charged with kidnapping, robbery, rape and forced oral copulation.
Police said that in the two incidents for which Jackson has been charged, he allegedly used a handgun and an electronic stun device against his victims.
Jackson was arraigned after he was brought back to Alameda County and is scheduled to return to the Hayward Hall of Justice on Tuesday to enter a plea.