According to this section in our Facebook Platform Policies under section II:
You may cache data you receive through use of the Facebook API in order to improve your application’s user experience, but you should try to keep the data up to date. This permission does not give you any rights to such data.
IANAL, but my interpretation of this seems to be that you can cache data but you do not have the right to store data in a persistent manner.
Another question to think about is how would you maintain consistency with the user's actual friends list. If you did store the data, and then the user adds 5 friends but removes 3 of his old friends, how are you going to update and keep the data fresh in your local database? You would have to pull the user's friends list from Facebook, which kind of ruins the whole point of storing it if you have to fetch it every time anyway (but since you didn't state the purpose of doing so, I assume you have a good reason for storing this rather than just fetching every single time).