I'm not worried about SF. The boom will be over soon. After the last dot-com crash, about 40% of the twentysomethings left.
Twitter is so highly visible in SF that when they tank, it will shake up the real estate market.
[1] https://www.google.com/search?client=ubuntu&channel=fs&q=TWT... [2] http://variety.com/2016/digital/news/twitter-declining-month...
people keep questioning this year after year but the writing is on the wall.