First thing that caught my eye was this paper: http://arxiv.org/PS_cache/cond-mat/pdf/0403/0403299v1.pdf

This sentence that caught my eye from the paper "humans spontaneously form groups of preferred sizes organized in a geometrical series approximating 3, 9, 27,..."

Seems to be at odds with the way current MMO's setup guild structures and group sizes. In some cases games may require a group of 10 people to sign a charter to form a guild, or a group of 5 to enter a dungeon. MMO's seem to force people to group in multiples of 5, when the natural order of things might make things easier.

The paper goes on do describe the different social group size and the varying levels of trust in those groups. The smallest core group size consisting of 3-5 people is called a Clique, this tends to be the people your most like to go to during sever distress.

Now your probably asking where I'm going with this.. From personal experience, players tend to stick with a game longer if there friends are actively playing. Those friends tend to be part of a players core group. If for instance we change the guild charters so it only requires 3 signitures, then we may be able to astablish and reinforce core groups. 

Military group structures as seen here: http://www.army.mil/usapa/epubs/pdf/p10_1.pdf on page 53 serve the same purpose. As guild gets larger coherence tends to get more difficult. As an individual in a large guild your role starts to seem less significant.