There used to be a lot of frogs in the city.  This mission local article says the mission used to have a lot of frogs:

But in the springtime between May and March, along the banks of old Precita Creek, long buried under Cesar Chavez, in a few places the distinct sound of frog can be heard. The Pacific chorus frog once lived all across San Francisco, and now lives wherever it finds good, neglected weeds and water that never seems to quite dry out.

On April 17th, 2012 really strong frog sounds could be heard at 24th and Capp around 10PM.