This is what young people do; they adventure. Then, many times, they come back to their roots to settle down.
I'm sure there're a few people around these parts with anecdata regarding the joys of finding housing and raising a family in the SF Bay Area.
I think this is a rather unfair, and possibly dangerous, attitude, to simply dismiss this phenomenon as typical young people "adventuring". Is it really adventuring if it's a decision made after countless hours of carefully weighing different employment options and coming to the conclusion that the US offers far more opportunities and far better rewards for our careers?
I have nothing but anecdotes to support that the brain drain is actually happening at any significant scale, but it's certainly not difficult to imagine why young people who are offered a US position would come to these same conclusions, given the sorry state of the Canadian tech industry today.
RE: Settling down. Seattle, which is a 3 hour drive from Vancouver, offers about double the average Vancouver salary with comparable housing prices. Needless to say, I'm not planning on going back until the situation improves dramatically.