Oh c'mon; they're not "leaving San Francisco", they're just dumping one of their many buildings.
From your link:
Google will keep workers in One Market Plaza’s shorter Landmark building, Lamont said, which is connected to the Spear Tower; together with another tower they make up the plaza’s complex. Google plans to redirect employees who work in the Spear Tower office to other offices in the city. Google also has offices at the Hills Plaza complex by Rincon Park, 188 Embarcadero and 215 Fremont Street.