If only I had a paid a little more attention to any of the Facebook is growing more quickly than the Universe is expanding articles of late, I might have caught a bug in my app before I received multiple one star ratings.
That said, live and learn and be sure to create the uid column in your users table as a bigint.
t.integer :uid, :limit => 8
I will never know how many countless Facebookers with excessively large uids tried to play my game in vain, only to watch as MySQL mercilessly truncated their unique identifier to the eighth Mersenne prime.