Storing Facebook Data

I know there have been numerous questions regarding Facebook's policies in which we can store information into our application's database. I have looked at Storing Facebook API data and also have looked at http://ift.tt/KHzX9P but I just want to ensure that I am not violating Facebook's terms of use. I want to store a user's objectID in order to get certain information about them for use on another screen on my application (namely their profile picture) into my database. I was wondering, is that considered okay? I have looked around and haven't seen anything explicitly speaking about storing a user's objectID so I just wanted to confirm this would be okay. If this is not okay, would any one have any suggestions on how I can get a specific user's profile picture? Thanks!