Figuring out who is on top of the heap in the online directory business is a complex task: On one hand, a “directory” category is kind of an oxymoron, given that so much of what is quantified is based on search engine optimization.

Only 25 percent of Citysearch’s traffic, for instance, comes from its destination URLs. Some sites, such as MerchantCircle, barely have a directory — they’re mostly about being discovered by a search engine.

Yet we have still attempt to label “top directories.” Obviously, these leaders are going to be the masters at both optimization and destination.

According to Quantcast, here are the leaders, in order:

1.    Yellowpages.com

2.    Superpages.com

3.    Citysearch.com

4.    Yelp.com

5.    Merchantcircle.com

6.    Local.com

7.    Urbanspoon.com

8.    Insiderpages.com

9.    Yellowbook.com

10.  Servicemagic.com

IAC’s aggregated share in the Quantcast report is of special note, given its ownership of Citysearch, Urbanspoon, Insider Pages and ServiceMagic, and its investment in MerchantCircle.