There are a lot of vacant buildings being sat on by property speculators, with very little property tax to motivate use thanks to Prop 13 (and because existing tenants hurt resale value).
Uh, this definitely falls under "citation needed". I've lived in SF for the better part of a decade, and I've never personally seen a vacant building that wasn't vacant for a reason (i.e. condemned, slated for demolition, etc.)
These kinds of stories are apocryphal.