These blog popularity graphs are based on number of incoming links and number of members who have made them as favorites respectively.
Notice the steep climb from position ten and above in both the figures where the slots are occupied by the who's who of blogging community.
And it's almost flat from position 50 onwards implying how tough it is for blogs in that region to maintain their current position in the popularity charts.
Pete Johnson, Chief Architect of HP.com, draws an interesting comparison between these blog popularity charts and the Long Tail curve of Chris Anderson - "there's plenty of ad revenue for the right suitors in that flat tail". True.