As someone who also has never lived in SV, I'd imagine that, if the majority of these people leaving are young, active, relatively wealthy individuals, that they would move to areas where they can advantage of that position. A lot of people in that demographic are probably relatively active and might have would want to move somewhere with access to lots of outdoor activities. My guesses would be: SLC, Denver, Phoenix.

Disclaimer: I'm from the Denver metro so am obviously biased here. Also please don't move here because housing is expensive enough ;)