historically there was a lot of media and some internet in sf. more interesting hardware type tech companies generally clustered in the south bay with some exceptions (oqo, sega).

twitter was notable because they put a big campus in midmarket... but there was plenty of internet and multimedia that preceded them. (organic, macromedia, razorfish come to mind but there were countless others)

sure, no sun or apple, but let's be clear, twitter was no sun or apple either.