As Joel said, you can use FBProfilePictureView
to get the image. If you don't want to use the Facebook API, you can use this really (too) simple method, just to understand the mechanism :
// Creation of an UIImageView (which will be used as the button's background)
UIImageView *imageView = [[UIImageView alloc] init];
// Fetch the image data from a specific URL
NSString *imageURL = @"yourProfilePictureURL";
NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:imageURL]];
// Assign the image data we fetched to the UIImageView
imageView.image = [UIImage imageWithData:imageData];
// Creation of the UIButton (if you didn't do this in your story board)
UIButton *button = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
// Set the background image of the button with the image you fetched before
[button setBackgroundImage:imageView.image forState:UIControlStateNormal];
// Add the button to the view (useless if you used storyboard)
[self.view addSubview:button];
As I said, this method is pretty straightforward, understandable and easy to use, but I suggest you to use the Facebook API instead (It will require much more work). See the FBProfilePictureView
documentation, then here and here for already asked questions about this class.