I am someone who has stayed behind, and works at a startup. As a full-stack web (Rails+JS) developer I certainly haven't seen any shortage of demand. Although I would say that about 25% of the recruiting messages I receive are from US companies - often acknowledging me being Canadian and in some cases even playing off of it with message subjects like "Hate the winter? Come south" or what have you.

There are some distinct advantages to staying up here though. First, at least in Toronto, the cost of living is substantially less than that in San Francisco. To maintain my standard of living, I estimate that I would need to earn 35 - 40% more in Cali, which would be at the upper end of most salaries I've seen. Second, "standing out" career-wise is much easier when there aren't 100,000 people with a duplicate skill set.