I wonder how many of the employees themselves are not eligible to vote (H1B/etc.). A lot of the political power of a big employer comes from two things:
1) Large voting bloc of employees
2) BATNA to getting their way: credible threat to move. Google can't credibly threaten to move out of SFBA, sadly, and Twitter couldn't credibly threaten to move to Brisbane (because, Brisbane is crap). If Twitter hadn't had crazy political connections, mid-market wouldn't have happened.
You don't get large amounts of power just by paying a lot of taxes or employing a lot of people; if there are big voting groups who dislike you, you're still screwed.