There's an option to disable auto-syncing as they call it, and it's on by default. Interestingly enough, when I went to check out the settings area just now, I saw several Twitter friends who I wasn't friends with yet, despite having auto-syncing turned on, so this looks like a bug on their end. I got an error when I tried to check my Facebook friends' status as well.

There's also an option to toggle being notified when a friend joins (presumably this refers to auto-syncing), which is on by default too.