Hey so I think you are missing the point when you ask this question. Lets take an ATM for example. You need your bank card and a 4 digit pin. 4 digit pin has 10^4 possibilities and could potentially be guessable.
Also its important to look at the bits of information you need. The card falls under "Something you have" and the pin falls under "Something you know". This is important part of 2 factor auth because it makes it highly unlikely you will aquire both
the problem with trying to crack the pin is there are other security mechanisms in place. At the ATM they use a throttle where you can only attempt to log in X number of times before they take your card. On the internet they will use a throttle and after a few tries they will force you to enter a recaptcha or they will have a required delay between failed attempts (a few added seconds before the page will let you try again) or just lock you out of the account
So....to answer your question. 2 factor auth might be crackable but there are steps taken to prevent you from cracking and make it extremely unlikely that an attacker will crack it. I don't understand your question very well so I went with this generic answer
edit: I looked at facebook like you asked. The pin they send you is 6 digits send to your phone via SMS. They do allow you to download an app to generate them on your phone which could be interesting to look at. They don't throttle logins but they do give you a "Facebook Notification" saying someone has tried to log into your account unsuccessfully.
Not Surprisingly you must have the correct password to get to the point to enter the pin.
If you are trying to crack it you are going to have to send a lot of traffic to Facebook and they will notify the person you are attempting to crack. Also, the person will get a text message with a code saying that someone has attempted to log in (with the correct password).
I would say as an attacker.....their facebook probably isn't a good place to go after remotely if they are using 2 factor authentication. It may actually be easier just own their box first and go to facebook from their computer that they have already logged in from.