Really, all Facebook wants is the old rail bridge with bus lanes on it, so they can draw more employees from the lower-cost (relatively) housing in the East Bay using their shuttle bus network. Rail (high speed or otherwise) is optional. Facebook is also building a company town on the Peninsula, keeping their options open.

As for HSR via Dumbarton & Altamont - I like the idea, but don't think it's practical. Too many NIMBYs along the way, so you would have to not only tunnel Altamont but also the Berkeley Hills, Pleasanton, Livermore, Fremont, and (East) Palo Alto. That last: there's a reason why there's still no freeway connecting 101 to the Dumbarton Bridge after all these years.