The other answers look pretty reasonable, but I wanted to add a few possibilities.

If an attacker recovers one or more of your passwords from other breaches, it seems like plugging them into Facebook's UI could leak at least a few types of information:

• Whether you used the password on multiple services or not.
• With known disclosure dates for the breach, they could also gain a data point on how long it took you to update the password.

If pattern-matching on recovered passwords suggests a likely iterator or mutation pattern in your password, it also seems like an attacker could also try one or two passwords they haven't seen in order to support or refute a thesis about how your passwords evolve over time.

I would guess any attacker is somewhat limited in how many such tests they could attempt without someone noticing, but it seems like information along these lines could inform heuristics for how known users change their passwords, improve statistics on how common different update patterns are, and help hone priority lists of users/accounts that are most vulnerable.

These seem like fairly small risks at the individual level, but it also seems like (Martin Bos' post on cracking hashes in the LinkedIn breach is instructive) small heuristic improvements turn into pretty nice levers for recovering more passwords in a large scale breach while simultaneously improving the heuristic/lever.

The other answers look pretty reasonable, but I wanted to add a few possibilities.

If an attacker recovers one or more of your passwords from other breaches, it seems like plugging them into Facebook's UI could leak at least a few types of information:

• Whether you used the password on multiple services or not.
• With known disclosure dates for the breach, they could also gain a data point on how long it took you to update the password.

If pattern-matching on recovered passwords suggests a likely iterator or mutation pattern in your password, it also seems like an attacker could also try one or two passwords they haven't seen in order to support or refute a thesis about how your passwords evolve over time.

I would guess any attacker is somewhat limited in how many such tests they could attempt without someone noticing, but it seems like information along these lines could inform heuristics for how known users change their passwords, improve statistics on how common different update patterns are, and help hone priority lists of users/accounts that are most vulnerable.

These seem like fairly small risks at the individual level, but it also seems like (Martin Bos' post on cracking hashes in the LinkedIn breach is instructive) small heuristic improvements turn into pretty nice levers for recovering more passwords in a large scale breach while simultaneously improving the heuristic/lever.