I thought my answer was pretty easy to interpret, but I will spell it out: Because the vast majority of people who use Facebook enrich their lives with it.
Spelling it out even further: Just like Facebook employees, the people who work in salt mines, or build swimming pools, go to work each day because they think about the vast majority of people satisfied by their product, not about the small minority of people injured by it.
I don't work at Facebook, but if I did, the answer to "How do you sleep?" would be "Like a baby."