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I work there. They've expanded a lot. If my reckoning is right Google occupies 8/15 floors completely, and occupy fractions of a couple more floors.

As of last year (when I worked there), 8 was a pass-through floor mostly occupied by ISPs. 15/9/10 should be done by now, though.

Is the rest of it empty or do they rent it?

Their tenants include Nike, Deutsch Inc, Bank of New York and Barnes & Noble. They also donated space for Cornell NYC Tech to hold classes there.