Facebook's motive in this particular venture is anything but an agenda to submerge in the mainstream. The Facebook office in Hyderabad is a operations center that will supplement operations in other support centres. It will also house online advertising and developer support teams. They have not announced any serious development teams for the Indian office as of now. Thus any Indian city will do.
Hyderabad too has its fair share of tech companies with it being first in the race towards attracting them (during the time of Chandrababu Naidu's Cyberabad) than even Bangalore. Google, Microsoft, Amazon, Oracle as well as Indian IT firms like Infosys, Wipro and TCS too have their offices in Hyderabad.
Besides Hyderabad has greater area to expand than Mumbai and until recently the State Govt. was more supportive of allotting land at cheaper prices to large tech companies than the other cities mentioned.