That's only one portion of the costs. Besides the associated taxes and insurance they have to pay rent/utilities, purchase equipment, etc etc. The quoted figure was total expenses / employees, not employee compensation.
That sounds right for direct staffing costs (health insurance isn't cheap!), but low if you mean counting in all the other stuff. I suppose it depends on the business--someone like Dropbox spends a lot more on servers than say Groupon.
Right, that's for the incremental cost of each employee: taxes, benefits, equipment (for the individuals, not servers), office space. It's useful for calculating how much a new employee will cost you.
I've always used a rule-of-thumb of a 100% overhead rate for employees - it's a spectacularly easy number to work with mentally, and its right to well within acceptable error bounds an awful lot of the time.
If I'm paying you $90k, I'll expect to incur an additional $90k in costs resulting from employing you.
