Last year two Harvard physicists published a breakthrough paper on the Cheerios Effect, which states that "Objects that float at the interface between a liquid and a gas interact because of interfacial deformation and the effect of gravity. We highlight the crucial role of buoyancy in this interaction, which, for small particles, prevails over the capillary suction that often is assumed to be the dominant effect."
Or, as the Wikipedia folks put it, it's "the tendency for small wettable floating objects to attract one another."
I think this works for large wettable objects, too, as witnessed by the recent influx of dad groupblogs: Dadcentric, TheBlogfathers, Dadbloggers, and Rice Daddies. Of course, Dadbloggers could also be attributed to the propensity for southern Christians to glom together, and Rice Daddies, well, think about it. Asian rice IS stickier...
All in all, more research is required.