Optical circuit switches with fast switching times can provide rapidly reconfigurable bandwidth in data center networks. Silicon photonic switches are attractive candidates with their fast response times and CMOS process compatibility. In this paper, we report on a 50 x 50 silicon photonic switch with MEMS-actuated vertical adiabatic couplers. The switch is monolithically integrated on a 7.6 x 7.6 mm(2) chip. It has an on-chip insertion loss of 8.5 dB. Our switch exhibits a sub-microsecond switching time (0.85 mu s), a broad spectral bandwidth (1400-1700 nm), and digital switching characteristic. The switch architecture is highly scalable as light passes through only one switching element irrespective of the switch size.