It might sound like a security hole to you. But trust me, It is one of the strongest security pattern.
It prevents you from avoiding you to create an old password of yours even in your hurry times. It is really necessary for you
not to re-create your old passwords, since there is always a chance of some people knowing your history(friends or foes, it still
counts as a threat to your privacy).
You have no idea how useful it is to an user when they are notified that they entered their old password by mistake OR they changed it this
time back,etc., It will really help a user who genuinely typed his old password by mistake.
Just by knowing that you changed your password last month will not help an attacker to guess your current password. It is based on
the information that you put out there online.
For example, when an attacker comes to know that you changes your password two days before, he might
check out your blog or social network sites you are involve in. If you put any related info that could lead to guessing your current password, thats when
you are officially busted and it not because of your networks but by you.
So just be careful with your online data. Update strong passwords frequently and bother less about your old passwords stored by Facebook,Google,Twitter. After all they exist to make money by your usage which means they are desperate enough to secure your identity and information as they secure theirs!