Good question :-)
The list you can see here is based on the combined factors of activity in Yedda: the number of questions, answers and other activities you can do. I'm not sure this is the best list we can come up with to point out the most active Yedda members. What would you like to see in such a list?
P.S. You might find the list of top ranking people also interesting, as it reflects the accumulated number of thumbs up a person was granted. Again, if you have any feedback concerning this list, I will be more than happy to read it.