It was pointed out to me in a few comments that the analysis of twitter communities for AFL and NRL followers graphed below miss an important dimension: time. So, here are the same graphs but animated to show the development of the retweets between account in time.
AFL Twitter Community
NRL Twitter Community
A few things are clear from the addition of time as a dimension in these graphs:
1) There are clearly events (probably actual game moments) which trigger multiple tweets at the same time. The moments are interspersed with long periods in which nothing seems to happen.
2) Retweets follow the original tweet very quickly. This makes sense in that twitter is a medium for communicating the present moment.