This may be a catchy way to make your company known, but I imagine you're only getting those who were thinking of leaving anyway.
Seriously though, to your point, why do you say "but"? Are the ones who think of leaving somehow less valuable than the ones don't think of leaving? In my mind, those who never think of leaving probably tend to be the weaker talent. That could just be my unsubstantiated bias, but either way, I think the point is that they all got hired by Facebook/Google at some point which is a pretty good signal as far as tech signals go.