How do tech giants manage turnover costs?
Companies like Google and FB are talent focused. Turnover is high for a few reasons:
 1). Because the employees tend to be VERY good at what they do, thus their opportunities are vast; and people tend to gravitate towards who pays the most.
 2). Stress. 
 3). Contracts. Many employees in these big firms tend to only have short term contracts for specific projects. 
 4). These companies demand some of the best talent and work, and if an employee isn't reaching par consistently, no reason to keep them.
I'm sure there are more reasons, but those are researchable. =-) 

Hope this helps.