If Plurk has 5 developers serving 1 million daily users, and Twitter has 1000 developers serving 100 million daily active users, you only have to believe that scaling by 2 orders of magnitude is twice as hard for it to be a tie in terms of developer:user ratio.
(I suspect the write load is rather different - the figures I can find for Plurk suggest 8 billion messages in total - Twitter writes half a billion a day.)
(I suspect the write load is rather different - the figures I can find for Plurk suggest 8 billion messages in total - Twitter writes half a billion a day.)