How often are Facebook servers being hacked?Depends on what you consider to be a hacking situation:
If you mean hack someone's account, then a lot. But this, almost every time, is due to social engineering, not because of some vulnerability on Facebook.
Be the other hand, if you consider exploitation of vulnerabilities on Facebook code/server, it still happens but much much less times. Notice also that these vulnerabilities differ on severity. You can check some of the people rewarded for finding these vulnerabilities here: Facebook
thanks for the A2A, my answer is the same as Kim's, while the question is not necessarily specific as to which aspect of facebook is being hacked, I've never heard of anyone successfully gainining unathorized entry into the facebook servers or system. But if you are referring to facebook accounts they are hacked all the time. The point of failure here isn't necessarily Facebook themselves, as they do quite a bit to protect your account security, but when users pick easy passwords and have secret question answers that can be learned from your own profile, its not making it a challeng to have someone who is up to no good swipe your account.
Probably a few times a year at most and once in a few years at least (see article for a 2012 hack).
But that doesn't include all the numerous NSA backdoors and hacks of FB for surveillance purposes.
British student jailed for hacking into Facebook | ZDNet
Snowden: NSA uses fake Facebook to hack into users' computers
At this stage of the game, it doesn't really matter whether or not he's cheating or she is cheating. You suspect that he/she is. Whether you're right or wrong about that, you clearly don't trust him/ her .So the lack of evidence proves nothing to you.Then, you'll conclude that he's even craftier than you thought, and he's covering up too well to get caught.....
What do you mean by "hacked"?
If you refer to a serious intrusion in Facebook's servers resulting in large scale data theft, as far as I know that never happened.
If you refer to individual people getting their account compromised, according to some documents released by Facebook a few years ago the figure was 600000 accounts per day in 2011. (Comments by Facebook PR people mention that this is an upper bound, including every and all kind of "suspicious" activity, including some false positives.)
AFAIK Facebook (the system) has never been cracked.
However Facebook account passwords have been hacked frequently, often with phishing attacks and in some cases due to other systems being compromised and users using the same credentials for multiple services.