According to a security document from Facebook, the company stops over 600,000 hacker attacks where hackers try to gain control of one or more accounts. With more than one billion logins per day, the number of attack attempts is approximately 0.06 percent.
Facebook also works hard to keep spam away from users. Compared to e-mail, where 89 percent of all messages are spam, Facebook has managed to keep it below four percent on the service.
The security document shows that each user has an average of 130 friends and combined users spend 700 billion minutes on the service each month.
You can find a detailed description of how security on Facebook works here.