These nesting sites were found between the years of 2010-2017 mostly by myself, but with contributions from probably a dozen other birders and interested folks. Not all the sites listed above are necessarily active from year to year - nests get removed, nests are abandoned, the site which the nest was built on can go away, and ravens can just decide to nest somewhere completely different. Also as the number of sites has grown, I've been unable to go back to check each and every nest from year to year. But a good portion of the nesting sites have been active across multiple years.
Also note that while I have walked, ridden, and driven many streets in San Francisco, there are still some neighborhoods that I have not fully explored (or perhaps not explored at the right time). Undoubtedly, there are then nests that I have not found.
The first nest I found, I stumbled across. I was riding home from work and noticed a raven by the Federal Building downtown, and was startled to see it fly into a nest on a Federal courthouse. I followed this pair through the fledging of 4 nestlings, and came back the next year for more. They did not return, but I found them (or a pair like them) nearby on City Hall, and then another and another. Since then, it has been an excuse to explore San Francisco's neighborhoods and most excellent geography, and I've become quite adept at sussing out nesting sites. Each year there also seemed to be a story written about the explosion of crow and raven populations, but it didn't seem like there was any data beyond the christmas bird count.
My estimates of the number of nesting pairs has grown each, as a realization of the size of the city, and the number of places ravens could nests has grown. Based on my collected data so far, I make some rough possible estimates here.
* I say nesting sites rather than nests because a raven pair -- in particular those on buildings -- will build a new nest near the old. The next year they might build another, or come back to the original. Sometimes they build more than one before they decide on which to settle. Nests built in trees seem to be more consistently used, though it depends on the type of tree (nests in eucalyptus tend to change from year to year)