Like what? Well, there is a geographic component (often), a time component, the textual content of the message, and the ontological nature of retweets. Gold! While each of these are valuable dimensions for illustrating data, the combination is so much sweeter than the sum of their parts.
Here's a recent more example using tweets containing the word, "torch." I've zoomed in on London to find a pretty well defined cluster of folks noting the passing of the Olympic torch on its way to the opening ceremony. The intersection of space (lat long) with time (tweet timestamp) shows a clear trending of a phenomenon (keyword) with more insight than those three can independently (check out this other, probably tighter, example). Even just a basic handling of those dimensions, like in the following snapshots...
Thumbs scramble to tweet descriptions of the "torch" passing through the streets of suburban London. A distinct geographic pattern appears. Using relative age to color the tweets reveals a vector from west to east and a relative frequency through the timeline (you may notice my local EST offset as I observe from across the pond here in Lansing, Michigan).
Here's an illustration of how interaction with a data element causes instances in other visual dimensions to pulse, providing a link between the three.
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